Problems with a wireless router on Blueyonder

Kind people of uk.telecom.broadband, I have a puzzle. (Yes, it's another
of those posts about problems with setting up a wireless system.) I have
a Blueyonder broadband connection, but want to share the connection with
my flatmate using a wireless router.

So, everything works fine with the broadband plugged into my new Dell
computer (running Windows XP). I install the router (a Dell TrueMobile
2300 with its own software) and that works fine with the broadband
plugged into the router and the router plugged into my PC.

The next day, I turn everything on... and it's now not working in that
configuration. I tried uninstalling the router software and re-
installing it. The installation programme works fine, it appears to
assign IP addresses appropriately, until the very last stage when it
checks for network connectivity and can't find any.

I registered the router's MAC with Blueyonder. No change.

What's going wrong? I was surprised that it should work one day and not
the next: does that imply it might be some simple fault in the new
router or wiring...?
--
Henry