The engineer came round first thing this morning and was good as gold (if a bit talkative - I'm very antisocial). He narrowed the problem down to the wiring between the outside terminal box and our main socket. Apparently, if the exchange detects any sort of fault on the line it automatically slows down your broadband speed. The reasons for doing this went over my head but presumably it's a way of rationing out bandwidth or something.
Anyways, the initial signs are encouraging and hopefully it'll be back up to its normal speed in a couple of days and not a moment too soon - I never did buy a jazz mag.