Well, not the whole of Cheltenham, just the bit where my servers are hosted. About 5 minutes after I lost contact, I phoned them up - they knew about it and were on the case. Some problem with a fibre. It was fixed by the time I woke up this morning.
We reached 2000 pageviews of this blog!
... later ... They told me what went wrong.
They had two separate connections to Telehouse in London, which should mean that if one breaks, the other is still running. Unfortunately, they both went through the same conduit, and when the guys at that end did some maintenance/updating, that took both of them out. There's a lesson there somewhere. They're looking to add a third connection now.