I did some more tweaking today, and can report that seemingly small changes (in fact changes that I wouldn't have thought would have made a jot of difference) can improve things, if, like us, you're lumbered with pretty slow ADSL.
I swapped out the old Netgear wireless router I was using temporarily (it was donkey's years old, dating back from when we had a Virgin cable service at our old house, which itself was truly awful). A newer D-Link one is now running things and although the speed didn't change at all, the D-Link does authenticate when you power things on a great deal faster than the Netgear; I've no idea why.
The surprising speed increase came from following a tip on the "Mr Telephone" YouTube channel. His advice to swap out the ADSL RJ11 cable for a cable made up from Cat5e or Cat6, with an RJ45 on one end and an RJ11 on the other seems to have given me the best part of another 1Mb DL. Before trying it I'd have said it was snake oil, but I can say for sure that making up a custom cable like this does seem to make a difference.
The new cable is only around 300mm long, and has an RJ45 on the end that plugs into the NTE5 Mk3 (although these take an RJ11 normaly, they actually have an RJ45 under the sliding lid). It's a bit of a fiddle getting Cat5e cable into an RJ11, but a bit of perseverance paid off.
So, the system now has an NTE5 Mk3 connected directly to the "jelly" underground incoming cable, with a Cat5e socket fitted in the wall immediately above, that runs through to the "computer room" (my study!). Above that I have a new, unlocked, HG612, running in bridge mode only, as a modem. That's powered over the ethernet cable, from a 12V battery-backed supply in the "computer room" The patch cable from the Cat5e socket to the modem is only about 150mm long, so with no power supply wires or plug-in supply it looks nice and neat, and doesn't use up a wall socket.
The D-Link router does the PPPoE authentication and then connects to the home server a wireless access point and a 12 port unmanaged gigabit switch. The switch handles all the connections to the rest of the house, via a patch panel on the wall.
All the low voltage devices (HG612 modem, D-Link router, 12 port switch and wireless access point (there are two in the house at the moment, to get good enough coverage) are powered by a home-made low voltage supply using PoE. That power supply is battery-backed and can deliver both 12V and 5V from high efficiency switch mode regulators, and can charge the battery at standby rate.
I've found that just using the spare pairs in the ethernet cable works very well for the low current all these devices use, so I just inject either 5V or 12V into the appropriate lead, using the doubled up brown/white-brown for the negative supply and the doubled up green/white-green for the positive supply (all cables are wired to 586B). To save modifying units internally for PoE using this non-standard method, I just made special patch leads for them with a power lead and connector spliced in. It looks neat enough, at least SWMBO thinks so (but I suspect she just like not having wall sockets taken up with plug-in power supplies!).#
I have a date for getting FTTC now, 14th November, and have left the HG612 set in "all" mode, so all I should need to do to switch over is change the username and password in the router, I think.
Plusnet are indicating that we should get around 27Mb DL when we get VDSL, which will be a terrific improvement on the just under 4Mb we're seeing now on ADSL, after several days of mods and tweaking. It'll be interesting to see how fast the service we actually get will be; I think I've done all I reasonably can to make "my" end optimal.
One final thing, the HG612 and D-Link combination seems very much more stable than either the TG582 modem/router that the PhoneCoop supplied, or the TP-Link 8817 that I was using before. Both of those used to drop the speed randomly, very often, the HG612 seems to be rock-solid all the time.
Thanks again for all the advice.