OK, so we're back online again, after a fashion. NTL tech support was right; I just wish it hadn't taken them two days to tell us what was wrong. Basically, it's like this. If you've got interactive services on your TV, then your broadband is almost certainly also working, and the problem has to be beyond the set-top box. ie, a fault with our cable, not NTL's kit. So I found another (shorter) crossover cable and have attached the router to the settop box. This means that all the computers that have wireless access are now online again; which means the iMac and one of the laptops.
I'm now going to grab the e-mail, but if you've been waiting for email from me, that's why you haven't heard. In particular green_amber, I got your letter this morning and am working on it.