You'll be glad to hear that I solved the problem by hard-wiring the PC to the router, thanks to a £4.99 RJ45 cable from fleabay, a tent pole, some string, and my drill!
It works fine, so the issue was the wi-fi link, I reckon. I'm not going to look further, anyway.
FWIW the Tesco VoIP handset (rebadged USR 9600) is OK, for £9.99. Use it fine with sipgate, Tesco (Freshtel in drag) and Freecall. The skype drivers are on the USR site I understand. The Tesco system (like Sipgate) allocates a UK 01xxx xxxxxx number for free, which costs £20 odd on Skype I believe. I'm sending one back with my brother in law to Oz, so his mum can call him for free (as she has BT Option2), as she'll be calling a standard UK number.