Phew; I now have working home phone and network again. Much better.
But NTL are certainly no better now they're Virgin, and may in fact have managed to become worse. Their installer chap showed up mid-morning (having promised an afternoon visit, and in fact he was lucky to catch me in and not having dashed out to the tip), futzed about with various sockets, and instead of having everything working by the time he left he instead promised me that it would all start working soon – the network within an hour, the full range of TV channels within 24. This may or may not be a genuine consequence of otherwise sensible network architecture, and it may or may not be a deliberate attempt to arrange that the installation engineers are out of range by the time you find out things don't work, but it certainly has the latter effect even if it wasn't intentional.
So an hour later I still didn't have working network: Virgin's DHCP server was issuing me a temporary IP address from which I could ping anything I liked, and connect to their auto-registration web server, but do nothing else at all. Multiple reboots and various faffing didn't help; I tried going through their auto-registration procedure, which didn't help either. Eventually I resorted to ringing their network support, which is now on a premium rate line. It rang and rang and nobody answered. I phoned the main Virgin support line, who explained to me that ringing for ages is that line's equivalent of being on hold and I should have left it for ten minutes. (That actually makes some sense since it's a premium line – it probably means you're not charged for the time you spend on hold – but that only makes me more irritated about it being a premium line in the first place.)
Anyway. Got through to someone in the end. We patronised and counter-patronised each other for a while (I understand more about actual computer networks, he understands more about Virgin's horrendous hackware, both of us felt a need to explain things loudly and slowly to the other).
Eventually he made me shut down both the cable modem and the computer and then start them up again in that order. Infuriatingly, this turned out to fix the problem. It certainly hadn't fixed it the previous time I'd tried it. (I realise lots of people will say that and be wrong, but I hope at least some of my readers will respect my general competence level enough to entertain the possibility that I might be right this time.) Dad (who happened to be visiting today) suspects he did actually change something at his end even though he didn't admit it to me.
(It does, on the other hand, make me realise that there probably is quite a good reason for making their broadband support line a premium rate number: probably a hell of a lot of computer-illiterate users will call up with problems that do turn out to be their own fault, so it's not completely unreasonable to make them pay for the privilege…)
So the result of that phone call was that on the one hand I had (and have) a working network connection, but on the other hand I felt about as frustrated and furious and upset as I would have felt if we hadn't managed to get it working. This seems to be a particular talent of NTL/Virgin: as well as having things fail infuriatingly, they can even manage to infuriate you when things work.
Still. With any luck, by tomorrow the annoyance will be gone and the network will still be working. <touches wood>