OK the BT HomeHighway installation is completed. The engineer who was supposed to be there "between 8am and 10.30am" was in fact there between around 10am and 12.30pm - but what does writing off half of a customer's day matter?
Turns out it took him longer than expected because there was a DACS on my old analogue line which needed to be removed - and the old modem which I'd found lying around and had declared to be a 28.8K one after testing can in fact do 33.6K now it's got a decent line.
It seems that I get to keep all three of the numbers on the various letters I received. One for each analogue 'line' and one for the ISDN 'line'.
However, in spite of the fact that they told me last week I'd see all the incoming voice calls on the ISDN S-bus, so the computer has a chance to answer them and do voicemaily-things, that's not the case - only calls to one of the numbers appear on the S-bus, and the computer can't set its Outgoing Caller Identity to either of the other two numbers either.
Perhaps I would have been better off just getting an ISDN2e installation. It appears that BT are charging £88 per quarter for ISDN, but £39.99 per month for HomeHighway. So although I'd have to get a TA to allow me to plug in my old analogue equipment, I'd probably be better off in the long run, and I'd be able to play.
Fucking Useless Telco.