Well, my Internet access has returned, finally. It’s been off for a week or so, and I hadn’t bothered to go and fix it. There was always a chance that somebody had taken an exception to my downloading GBs of Linux ISOs and MS Visual Studio Express – and I didn’t really want to risk getting shouted at!
But instead it turned out that some peculiar part of DHCP had become broken -and now it’s all fine.
Hurrah!