Well that certainly covers the TV and FM side of things - the mast is absolutely bristling with antennae though so I guess there could be loads of other stuff up there too. There are also two smaller masts next to it.

Am off to bed in a second but just took a look at the router stats figure at the bottom of my screen, sitting on 10dB despite it being 21:40hrs - I think the RF3 is definitely helping. Router stats has shown that the SNR has not dropped below 9dB all evening. In fact its been hovering between 9 and 10dB all the evening since around 16:30hrs. As my router only displays snr in whole dB increments (no decimals) I guess that in reality the snr has actually just see-sawed slighlty around 9.5  but the rounding off of the value has caused the alternation between 9 and 10.
Errors are still really quite good too.
It may be a little early to celebrate but it does seem to be £7 well spent.

That certainly sounds promising, hope it continues like that for you.

Thanks Chris, its still looking good tonight too. Tomorrow evening will be 3 days stable sync so profile should be back to 2000.
I will leave it be over the weekend but if someone could look at droping my target SNR next week (assuming no disconnections in between ) that would be great.

Fingers crossed it stays stable for you, give us a nudge in here next week and we'll see what we can do about your SNR.

At 4 to 5 miles - the out of band noise from those broadcast transmitters will be fairly low power density, and the antenna multicouplers ( mix several transmitters for feeding to one antenna) will filter it out ( takes me back to when  I worked on the original ones at Marconi !!! ).

But as you say there is more than just broadcast transmitters  there - but they will be lower power.

And Cellfone 800/1500MHz - will likewise have little noise down at 1MHz

I'd suspect the street lamps or such other local noise sources

I dont think streetlights are to blame as noise figures picked up well (pre RF3) whilst the streetlights were still on around 06:30hrs. Possibly something else electrically noisey. All our mains power is overhead and in the field behind me are some 32KV power lines but then I thought they would put out the same noise all the time.

Anyway, the RF3 seems to be working, no sync drops, noise not been below 9dB (currently 10dB), profile now up to 2000 and low errors.

A BT speedtest has just shown a 1500kbps throughput so not too bad a speed either considering all the network managing going on at the moment.

If Chris works his magic next week with target SNR I will hopefully be putting this saga to bed.

You may have told us already, but wading through 7 pages isn't on my agenda right now...... 
Does your neighbour have a "modern" NTE5a (with Openreach logo), iPlate or filtered faceplate?
If so they will effectively have an RF3 on their line.

Insulators breaking down and leaking in the cooler damper conditions overnight? But if it was that I suppose you'd also get more noise during the day if the weather was wet or foggy.

Also it seems it doesn't affect the neighbour so it must be something fairly local to you.

Do you have anything metal near the router or wires that could be acting as an aerial (e.g. central heating pipes, brackets for shelving fixed to the walls, desk legs)?

Have you tried moving the router or wires to see if that makes any difference?

I do have an openreach badged NTE5. I dont have an iplate and I do have a filered adsl face plate.

I have taken the NTE5 apart and there are no coils in there, just a diode and capacitor on a PCB so no built in RF3 there. I haven't taken the face plate apart but I sont assume that there is an RF filter in there either since the results of using that or plugging a micofilter into the test socket are the same.

There are really no differences in layout between my setup and next door (the houses are mirror images). Neither of us have extenstions connected, both NTE5s are on an outside wall. No metal shelves or similar. Both them and I have a double socket a couple of feet from the NTE5 into which the router and phone base station are plugged in.
The phone doesn't affect things as I have tried switching that off and it makes no difference to the noise figures (and the only problem is at night anyway).

The one obvious difference is that my dropwire goes straight into the large black junction box at the top of the BT pole as do all the other lines on that pole (6 of them) whereas the line from my neighbour connects to a small grey box at the top of the pole which then connects via a short length of cable to the big black junction box. I have no idea what the grey box is or does.

Without  a doubt, and whether it should or not, fitting the RF3 filter has definitely stabilised my line a fair bit so I'm keeping it wired in. Once the target SNR gets dropped, I will see how the situation is. Hopefully if I still only get  a 3dB drop at night then it will support a 6dB target with low errors, especially if I aim to sync the router early evening.

I think I was suffering from late-evening brain-fade, it's the bell-wire filtering included in newer NTE5a's etc. not RF3 functionality.
Anyway, thanks for the info and sorry for the confusion.

Could it indeed be an RF3 in that box on your neighbours line?

@HP
Now, that's interesting  you should mention about the OpenReach NTE5a & I-plate, because they do have a common-mode filter in them, but as far as I'm aware the Filtered Faceplates do not have a common-mode filter, & Jules was already using a Filtered FacePlate. Also, it's that long ago since I saw an RF3, I can't remember what is in them or the spec.
@Mal
Whilst your theory is sound, you need to remember that the field strength could still be high enough that with a bad joint somewhere (which there may be on Jules line) or a small bit of poorly designed circuit, or a semiconductor device that happens to be sensitive to a given (band of) frequencies - strange effects can occur!
@Jules
You aren't looking in the right place, I'll see if I can find you a picture. The significant bit is that it is OpenReach badged not BT Piper badged, can you confirm about yours and your neighbour's NTE5. Also you didn't happen to mention about this double socket and the few feet of cable - how many feet and what type of cable is it? Nor did you mention about the 33Kv lines behind you where's Mand's persuading stick
I know you said there was no difference in the figures when plugging into the test socket, but all the figures apart from a few kbps difference in sync would have had to have been identical - but never mind now.
Jelv's points are also valid, might explain any unexpected variations (or spikes) at anytime of day - the loading on the powerlines will vary during the day (and hence the radiation), depending on what it's supplying.
Anyway, I'm keen to see your latest stats, can you post some current ones as soon as you can?

@jelv - yes indeed - we did discuss rusty bolt effect some time ago with another posters difficult problem. But you wouldn't expect intermods, etc etc from VHF/UHF transmissions to stretch down to 1MHz - at least not at the level to cause a problem on a balanced ( well almost balanced ) line.
But as we have both said before - radio interference is not an exact science all the time - trial and error are needed to resolve problems - if indeed it is that in this case.

Anotherone - Thanks for the info. My NTE5 is definitely openreach - it has raised letters saying openreach across the top of the front plate.
Not sure of nextdoors, BT piper I think but they are away at the moment so cant check.
I forgot about the HV cables until the other day sorry. Line runs across field behind me, about 300metres from my house. 240V comes in via overhead from a transformer about 600 metres away.If it makes any difference, earthing is PME (T-N-C-S), there is a pole in my garden with a PME earth bond on it and a big earth rod at the bottom of it.
Phone line comes from front of house.

Double socket is around 2 feet away from router. Wired into downstairs ring main with 2.5sq.mm cable and sunk flush into wall. Socket is steel not plastic face. Only things plugged in are psu for router and phone base station (and unplugging phone psu doesn't change noise figs). Cable from adsl filtered face plate to router is screened high quality one.

Inside the RF3 is a small pcb. there are three screw terminals, a three way idc and a small core with two copper coils wond onto it. My dropwire is connected to the screw terminals marked A and B, a short length of cat5 links the idc terminals marked A and B to the A and B on the back of the NTE5.

Current stats (17:45hrs and dark outside) :-
Connection speed 2496kbps (downstream) / 448kbps (upstream) - router resynced today for some reason - dont know why as wasn't here but very windy today so could be power related - previously 2304kbps
Noise 9.0dB (downstream) 14.0dB (upstream) - router shows 12.0db at time of reconnect so not dlm dropping margin
Attenuation 58.2dB (downstream) / 31.5dB (upstream)
Power 17.5dBm (downstream) / 11.9dBm (upstream)

Chris, I have sent you a message, please confirm if you get it and are able to action.
Many thanks.

I don't suppose Chris will see that until tomorrow.

I was just being a bit of a pedant about the double socket  I don't expect that it will have any effect whatsoever. But it's always nice to have as complete a picture as possible in these situations.
If I'd known about all the transmitters 4-5 miles away and the 33kV line, I think my view would have been shove some filtering in, and would have been much more upbeat about the benefits of an RF3. Well I'm glad it's made the significant improvement it appears to have.

I guess that because your overhead line is a different length to your neighbour's, it will be "tuned" at slightly different frequencies. I'd also guess that a lot of the noise may be coupled in from the powerlines, which themselves are nice antennae picking up all sorts, never mind the inherent problems that jelv mentioned.
As for the grey box I forgot to mention before, if it is about 5" across and 4" down and not quite rectangular, it will be just where some connections with jelly crimps are made (normally).

Let's hope things stay nice and stable. What sort of time did that last resync occur? Out of curiosity what are the current stats like?

No worries, the message for Chris is just about dropping my target SNR as he suggested a few posts above, just thought I'd send as a PM so he notices it rather than trawl through this rather long thread.
My fault entirely re transmitters and HV transmission, its like most things, you forget about local things that just seem part of the furniture. At least we seem to have got there in the end. I'm glad that with all the info you have come to the same kind of conclusion as me re the noise. I know I'm never going to get a blinding speed but enough to give a stable 2500profile will keep me happy.
(Checked mothers the other day, 8192sync, 15dB SNR and 2dB attenuation - shame she hardly uses her computer, hence on PN value)

I think you may be right about the box, it is certainly grey and around the dimensions you give, cant measure as its on top of the pole but looking at the diameter of the pole, I would say its about the size you give.

The last resync occurred around 13:05GMT on 21/11/2009 and it resynced at 2496kbps with 12db SNR.

Current stats as of 20:30Hrs 23/11/2009
Connection Speed 2496Kbps (Down) / 448Kbps (Up)
Noise Margin 9.0dB (Down) / 14.0dB (Up)
Attenuation 58.2dB (Down) / 31.5dB (Up)
Power 17.5dBm (Down)  / 11.9dBm (Up)