I've tried the usual trick of powering down the router, and leaving it for a while, before bringing it back up. It hasn't fixed it.
I'm in the Bristol area, and see that Openreach have declared "matters beyond their reasonable control" in respect of both repairs and new commissions, as of late last night. (recent gales and torrential rain during Storm Imogen).
I'm unsure whether my problem is likely to be coincidental (doesn't seem to be an outright failure of anything, and the phone is working fine), or part of the acknowledged issues in the area.
Edit: OK, seems to have settled, though I'm no wiser what it was. Have taken no further remedial action at my end, so maybe it was to do with local problems caused by the storm.
Edited by TLM (Thu 11-Feb-16 20:47:57)