Has my throughput been capped?

[#1]

Quote from: fester

Should the upstream be all over the place like this?

No it shouldn't. My immediate reaction was a bad joint somewhere.

Quote from: Townman

It rather looks like lower frequency interference.

Not at all.

Quote from: fester

I connected up a phone and still no difference, but I tried a quiet line test and there is a little crackle there.

Did no-one pick upon this? It seems not.

Quote from: fester

Right after I ran the QLT I disconnected the DECT phone from the power and phone sockets so the brief introduction has caused a permanent drop. Might be a weak joint?

As per previous comment, quite likely, but plugging in and removing a phone, and especially a DECT that's seen to have a visible affect on SNRM may well cause a sufficient enough spike to drop the connection on a longer line with a bad joint. Best not plug phones in and out whilst the modem/router is in sync, especially in this situation. Double filtering a DECT (but not the modem/router) can sometimes be helpful.
Forget the radio and trying to find interference, you will be chasing your tail indefinitely.
The spikes on the Rx SNRM graph ~2212-2233 also confirm a bad joint somewhere, until your line is fixed your broadband is going to suffer.

Quote from: fester

I divert everything to a mobile (have done for over 8 years) so I’m guessing calls are diverted on the network and never reach my line?

Really? You must be mad, you have to pay for the answered diverted calls (to the Landline provider) a mobile call rate at that, as well as the Call Divert facility, what possible reason might you have for doing that? I might be able to make some more helpful less costly suggestions. Can you confirm who you pay line rental to?
You are correct, the call, hence the ringing will never reach your line. Ringing current can be benefical in keeping joints clean but if they aren't too clever, it will cause a dropped connection, so I wouldn't try anything after dark, your SNRM will be down and you'll get a slower speed, nor would I do it initially with the modem/router in sync, and in the present circumstances you really will need to do graceful disconnects to avoid the further wrath of DLM.
I'll post back a suggested course of action in a bit, I want to fully recap on your thread.

[#2]

Had a quick recap, if you are away a lot, I can see the reason for the divert, but not knowing the circumstances, what you pay for it, whether anything is inclusive, what happens if your turn it off, do you lose some freebie etc. I'm not going to say turn it off to do some tests (in daytime) at this point. I'll post back more a bit later, so as you may respond in the interim, although as the thread hasn't been touched for over a week, I'm not expecting a reply. Nevertheless, even if you don't, I'll post up the suggestions, there may well be others with similar issues reading, and the suggestions may be helpful to them.

[#3]

Thanks very much for the info and advice.
I hooked up a standard corded phone and there was far less noise than the DECT. Tried it a few days after and it was same. I guess it’s 2.4Ghz interference with the DECT and router?
I don’t actually use my landline number, but I direct a 0845 number to it (for free) and then divert to a mobile. Plusnet divert costs £2.50 per month and 100 monthly min to a mobile (more than covers my needs) costs a further £3. It’s been very reliable over the years (BT previously) so at that price I’m not really looking to change. I might look into another option if I don’t need the landline for broadband at some point in the future, but for now I’m happy being mad 
I can disable divert for testing etc. but it would need to be switched back on. It may seem petty, but if the line now can’t sustain itself without regular ringing current it would probably tip me over the edge into another option altogether. I kind of grudge the line rental already so it’s bad enough I’m getting 10% of the national average speed… if I now can even use the divert option I’ll give 4g or satellite a try. Only reason I haven’t already is the usage caps and latency issues, but I might be able to manage with a free nightly usage option or even go for a pricy unlimited package.
I couldn’t find a ticket option and no response from Plusnet to the thread (or PMs) so I switched back to the Billion router and found a -7db tweak setting that took the SNRM from 9db to 2db and have been running with it since the 11th. Some strange activity over the period, but only one brief disconnection on the 19th. After this the upstream SNRM jumped to 10.5db and varied wildly. It lasted for about a day and then it was reset to a default of 6db. I don't know if this was an auto or manual reset.
I adjusted the tweak so it was sitting at 2db again and there has been 4 disconnections since the reset. It’s giving about 3.1Mbps sync and 2.5Mbps throughput. I have attached some individual graphs and a few group shots.

[#4]

Quote from: fester

It may seem petty, but if the line now can’t sustain itself without regular ringing current it would probably tip me over the edge into another option altogether. I kind of grudge the line rental already so it’s bad enough I’m getting 10% of the national average speed… if I now can even use the divert option I’ll give 4g or satellite a try.

Just to be clear, this is a diagnostic test to verify / eliminate a theory.  Ring current could improve things, equally it could force a disconnect.  Either out come points to a joint issue.
Out of interest, if you dial 17070 option 1 (ring back) does the line ring or does it divert to your mobile?
NB: If you get an engaged message, just call it again.

[#5]

No it won't divert to a mobile and it will likely cause a connection drop in the present circumstances. That is NOT a wise suggestion at all, and it also appears some extra emphasis is needed.

Quote from: Anotherone

Ringing current can be benefical in keeping joints clean but if they aren't too clever, it will cause a dropped connection, so I wouldn't try anything after dark, your SNRM will be down and you'll get a slower speed, nor would I do it initially with the modem/router in sync, and in the present circumstances you really will need to do graceful disconnects to avoid the further wrath of DLM.

[#6]

Quote from: Anotherone

...............but plugging in and removing a phone, and especially a DECT that's seen to have a visible affect on SNRM may well cause a sufficient enough spike to drop the connection on a longer line with a bad joint. Best not plug phones in and out whilst the modem/router is in sync, especially in this situation. Double filtering a DECT (but not the modem/router) can sometimes be helpful.

Re: the noise with the DECT, try double filtering it, ie. -
Filter 1 ---- (phone output) ---- Filter 2 ----- DECT
    |
Modem/router
If the noise you can hear is a quiet hiss, it may just the audio circuits of the DECT are a bit hissy. It's crackles, squeaks and pop type noises that we are listening for on a Quiet Line Test. BUT I would proceed as follows with a "Graceful Disconnect" before doing any further checks and tests with the phone, and do it well into daylight hours, because as you are on a long line, dusk to dawn noise will reduce your SNRM (hence speed on a resync) -
A "Graceful Disconnect" is log into the modem/router and click Disconnect to drop the PPP Internet session. Wait about 30 seconds or so and the pull the Power plug.
After about a minute you may unplug it from the line or make any other changes, swaps, plug in a phone etc. The thing not to do is just unplug the thing from the line before a power off. If the modem/router you are using has no Disconnect/Connect button, just pull the Power plug and then wait a minute (but then stay off-line for 10 minutes or so, whether testing or not)
So with the modem/router off, try the Quiet Line Test and then try the Ring Back Test 17070 option 1. Let it ring until it stops. Do the QLT again. If the line is crackly do the Ring Back again. If the line is noisy after that, try the Phone Trouble Shooter on here, hopefully it might find a fault and automatically report it.
If the line is quiet, you could power up the modem/router again, but leave the phone connected. Now depending on how many (if any) drops you've had recently and what your error figures have been like, it will be up to you if just want to now try a ring back test, I'd be inclined to look at the RouterStats SNRM graphs first, and if they looked reasonably clean, try the ring back test.
If all has gone well, and the graphs are looking cleaner and/or the troubleshooter found a fault, that'll be useful progress.
You could take the phone off divert, when you are home (but not forget to put it back on again) or use the ring back now and again, as an interim measure until a real fault is found and fixed and/or the connection starts dropping - if the latter, report a phone fault with noisy line.
Hope that helps with some improvement.