@Caerefail
Unfortunately you can't choose ADSL1/2/2+ on this restricted 2704n, and IIRC Plusnet can only set 21CN ADSL2+ or ADSL1. Has anyone from Plusnet confirmed that your connection was ordered as ADSL1? - because I don't believe that will have been done. - It's often done when a user has problems with the connection and raises a fault. As jelv mentioned in reply #3, the mode has settled at ADSL_G.dmt simply because of your distance from the exchange and tones above 255 not being usable.
What you are experiencing doesn't really constitute a problem, just a damned irritation. DLM isn't changing the Target SNRM.
As has been explained, you are most likely getting the large day/night variations in SNRM because of your long line - which will pick up MW/AM propagation from around Dusk to just after Dawn. Night-time variation can sometimes be gross, so I suspect your assumption that you are suffering REIN and SHINE isn't right.
If your daytime SNRM is below 6, then there's a good chance your night-time SNRM will drop too low on occasion that the connection will drop.
If your connection is only dropping every few days, then providing the ES count isn't excessive, DLM will not see it as a problem.

Quote from: Caerefail

Okay, it's got me confused then. I know when the router was turned off the other day it reconnected in the morning with a daytime SNRM of around 5dB, which is too low to maintain a connection with the nighttime noise that we get.  It was steady during the day but as expected, it dropped connection overnight and when I checked the stats next morning, after a deliberate graceful disconnection and reconnection, the SNRM was just over 8dB so something raised the SNRM. I know the daytime SNRM is pretty steady from monitoring it. Anyway, even with a daytime SNRM of around 8dB, router stats are showing an attainable rate of 6424, whilst the downstream rate, which I know the IP Profile is based on, is only 5504, so I was wondering if there was anyway to get the downstream rate higher. I've done a graceful disconnect and reconnect but it didn't have any effect - would a full reboot of the router make any difference?

A mentioned, daytime SNRM's below 6 will give problems. You mentioned about reconnecting in the morning, now if this wasn't a good hour after sunrise, then noise levels could still be varying significantly. There are odd days when it will still vary anyway at this time of year as AM propagation effects are slightly different. Monitoring the SNRM is the only way to be sure and you usually stand a better chance of it being stable nearer the middle of the day, and certainly a good hour before sunset.
It is possible that you might have an odd SHINE event in the night which might just take the SNRM too low when it's already in a dip and that's when the connection drops - events such as Economy 7 heating thermostats and so on might cause this - maybe not yours - these sorts of things are not that common though.
In your case you may just have one of those "awkward connections". What I would first of all do in your situation, considering your line location on the same pole as a street light, I'd double check that the light isn't chucking out REIN causing the SNRM to drop when it switches on. Look at your SNRM on the relevant stats page - one of them updates automatically every x seconds - I forget which, and wait for the light turning on at dusk and see if there is any marked change in SNRM. If not, at least that's eliminated a potential problem.
On a day when your daytime SNRM is nice and stable, I would do a "graceful resync" .You mentioned a graceful Disconnect and Reconnect but if that didn't involve powering off the modem/router, it was probably nothing more than just a Disconnect and Reconnect of the PPP Internet session which will have no effect on sync.
The technique for a "graceful disconnect (resync)" is log into the modem/router and click the Disconnect button to drop the PPP Internet session,wait about 30 seconds and then pull the power plug on the modem/router. Never just unplug it from the line to drop sync.
Wait 10 minutes or so and then power up again. If needed, login and click Connect to establish a new PPP session.
Immediately check the stats - if the SNRM is stable, that's a good indication of the current Target SNRM.
If the connections holds for several days and then drops one night, you can repeat the process. As previously mentioned, if it's only every few days, and the ES count isn't an issue, then DLM is unlikely to react.

Plusnet usually order lines with slow estimates as ADSL1. If it was ordered as ADSL2+, then it will either always run as ADSL2+ regardless of whether or any tones above 255 are used or not, or it will switch from ADSL2+ to ADSL2 if no tones above 255 are used. ADSL1 is rarely selected automatically if the line is ordered as ADSL2+.

If that's the case, would it possibly be modem/router dependant as to which mode is selected for some odd reason? I have seen 21CN connections switch to G.dmt (according to the modem/router) when tones above 255 are unusable.
Whichever, only Plusnet checking will confirm that, but in any case, isn't it all rather irrelevant? Caerefail has a perfectly good uncapped upstream speed, so that's not a problem, the issue is the SNRM getting too low at night on occasion, causing a drop in connection and a consequential resync at a slower downstream speed. The actual mode won't affect that.

It's not irrelevant if switching to ADSL2 will make the connection more stable and not drop out overnight. Yes, the actual mode could affect that.

Plusnet cannot set the connection to ADSL2 IIRC and you do NOT have that functionality on Plusnet's 2704n !!! So it's a bit academic.
Whilst I won't disagree that a modem/router with a powerful enough processor capable of doing sufficient bit swapping with the minimum bit load of 1 on ADSL2 might stand a better chance of holding the connection, it's by far from guaranteed. And to be honest, I don't recall any cases where it's happened in practise. Most cases aren't marginal. I certainly wouldn't consider spending good money on a modem/router with the capability of setting the mode just to test that possibility. If you happen to have one lying around, that's different.

@AO Thanks for your detailed explanation. Prior to switching to Plusnet, I did some detailed monitoring using RSL on a Billion router. No blips when the street light switched on but plenty of irregular spikes overnight, which, as you say, are enough, coupled with the normal increase in interference, to drop the connection if the daytime SNRM is 6dB or under. I've some suspicion that it might relate to next door's solar panel system but no proof. I think I'll just accept that I've got an "awkward line" - as I've said, I've no complaint about the speed compared to what it was 
@ejs Yes, it actually says on the Plusnet website that they'lll supply ADSL1 if the forecast speed is under 4Mb. As my forecast was only 2.5Mb, I got ADSL1. If the SNRM during the day is 7-8dB the  line is totally stable and I get 4.5Mb download - I am quite happy with this - I was just trying to squeeze a bit more if I could  ;). I am getting a very good speed for the length of line and looking at attenuation information, it seems unlikely that ADSL2 would be much better.
I'm not complaining at all - so far Plusnet have been great.

So what happened to the Billion and what model was it?

It's been a long time since I tested ADSL1 on my line, but when I did back on 26 May 2014, the interleaving on ADSL1 was less effective at correcting errors, resulting in far more CRC errors when some extra interference arrived, and it dropped the connection more often. ADSL1 did get a slightly higher speed. The longest uptime I've got on record is about 55 days, and that was when it was left to end up on ADSL2+ automatically, although part of the extra stability might have been due to ADSL2+ resulting in a slightly lower speed.

@AO The Billion was a 7700N. Given that the speeds were so low with the previous ISP, I decided to get Plusnet's router when I switched to them, as I wasn't completely sure that the Billion was okay, the idea being that I may switch back to the Billion at some time. However, although the 2704N is dire for monitoring, it's behaved very well in all other respects so, coupled with the speed increase that I've seen, I've not bothered to switch it back.

That's been a very sensible course of action. As you say, the 2704n is dire for monitoring (and a lot of other things in the router part as well) but the ADSL side is actually very good.
The 7700N is also a good modem/router and IIRC it has the ability to set Target SNRM offsets (ejs might remind us) I'd have to go and check the relevant data and I'd probably point one particular "expert" user of a 7700N, on a very "awkward" line, in your direction for tips on the best way to set it - I might be telling granny to suck eggs mind    - so it could be worth you considering giving it a try at some point in the future.
Your line might best cope with say a daytime 7dB Target, but the disadvantage of setting that in the modem/router is that if/when you happen to get an overnight sync drop, you'd probably get a slightly slower sync speed than currently, until a resysnc - but if it happens even more rarely, that might be tolerable. It's an approach I've used in the past (with a different modem/router).
If at some future point you decide to have a try with the other modem/router, don't forget to swap in daytime using the graceful disconnect/resync technique I described in reply #16.

Yes, you can telnet into the 7700N and set the SNR speed. I did try it previously but it didn't seem very stable and my gut feeling was that it was the router not the line - but as a change of provider was on the cards, I didn't follow it any further. Thanks for all your help and advice 

I think it's a case of getting the setting in the 7700N just right otherwise it can appear as you say. I'll follow that one up in slower time.

Mmmmm. Just done a graceful disconnect and resync. SNRM when I disconnected was 8 dB, very few errors on line and everything stable. Just checked the stats now and it's 5.6dB :(. Speed is obviously up but it'll be interesting to see how long the connection lasts. I was really hoping that it'd resync with an SNRM of 7-8dB. Maybe I should have done the resync in the evening?

Sometimes with awkward lines, that can be a more successful option where you have a modem/router that doesn't give the ability to tweak the Target SNRM, but the problem is that noise levels then are usually so up and down in the evening, it's a bit hit and miss as to whether you hit the right speed/SNRM combination.
It sounds a bit as though your local SNRM is going up and down a touch today - it might not be a good day for propagation - I haven't checked at all myself today. If your local conditions were more stable, I would have expected to see the SNRM just after a graceful resync to be around 6.3 +/- 0.1dB typically, but life isn't always as nice as that - the joys of a rural environment!