any error counters on the router ?

Update:

Download speed normal this morning - typical - BT engineer due today!

BT engineers arrived and did some fairly thorough testing, but couldn't find a problem (not surprising as it seems to be working today for the first time in over a week).

BY CHANCE they were speaking to a colleague who asked them where they were and then told them that he had (yesterday at 6pm) fixed a problem a few doors away caused by a faulty monitor power supply. Seems it had been causing them problems for a while...
Prognosis is that the faulty power supply was feeding back into the mains supply and causing so much noise that it was affecting our router

Without this chance conversation, we would have been declared clear and charged for the visit.

Seems BT Wholesale/Openreach/BT don't have the ability to see patterns/links/etc in problems that closely located customers ar having.