Thanks all for your responses. They're much appreciated.
I think, all in all, based on your responses and on searches in this and other forums (fora?), the balance of probabilities is that, on average, overall, I'm probably better to stay with Sky+ for now. It sounds like TV Drive still has a number of issues with the technology and I really don't want the hassle (plus the present Mrs rfctabs would not be impressed with engineer visits every few weeks).
I'm going to drop Movies and Sport on Sky and take the 4 bundles for £18 deal. As I'll no longer be on the full package, however, the £10 Sky+ fee will become payable, and the £10 multiroom will still be there, so my subs will be £38 - a £15.50 saving. I've also just dropped the phone from Unlimited to Evenings and Weekends, a further £9 saving. Total reduction £25.50 per month, or £306 per year. Should pay for a few curries

(or a new pair of earrings for the present Mrs rfctabs

)
PS swissle - when I called TW on Friday, the helpful operator gave me a monthly price including two TV Drives @ £10 each per month. I was quite clear that that's what I wanted. Maybe there's still some confusion on the ground about what can and cannot be done. Still, I won't be finding out for sure just yet.