I didn't know there was a Thomson Speedtouch wifi router - not seen it for sale anywhere.

In any case, I returned the 3Com router to Carrefour and brought home a USRobotics one instead, which works perfectly. And it was QR40 cheaper, and seems to have a stronger signal than the 3Com one did.

Thanks for the advice all round, and particularly to treasures1234 and Terramax for making it clear that it was the router causing the problems, not my settings.