Update: I actually had a follow up phone call from TalkTalk on Saturday. The caller seemed to think I'd spoken to someone earlier in the week but of course I hadn't. Anyway ...
Inevitably having told the caller the line was perfectly stable as far as I was concerned, I woke on Sunday to a line that had been restarted at about 3am that morning. It was now showing a max download of 37 with actual syncing of 32.2/6.9. I've been assuming these middle of the night restarts have been deliberately triggered by TalkTalk or OpenReach.
48 hours on that remains the status quo.
Thanks for the update. Just to confirm, has the line restarted since Sunday?
No it hasn't restarted since approximately 2:50am on Sunday.
Ok thanks for confirming this. Has the speed also remained consistent during this time?
Yes, I've never actually seen the WiFi hub change speed without a corresponding restart of the line, but then I've not had it that long.