From Friday mid-afternoon to Monday mid-morning, courtesy BTConnect. It took me 18 hours persuading six different 'help' people that I did not need to:
a) Reboot my router
b) Replace my entire system
c) Go to 'Start' on my PC (sure, it's a Mac...)
d) Check I was using exactly the cable which came with my router (no, it was 3 ft long and rubbish!)
e) Make sure I had not suddenly, spontaneously enabled a firewall which - unusually - blocked ALL traffic over ADSL
Eventually, after the third line check, the (least experienced script-reader of the lot) girl said - OOh, I have seen something odd in the report on your line check...
Supervisor called in, specialist team sent to investigate, of course they had screwed up something at the exchange and lots of people were affected. They were probably all going through the same thing.
So sorry about any delays in replies to PMs or posts, and I may have missed emails (unsure, it seems to have downloaded everything once service was restored).