Thanks for the effort... I tried from another PC and it worked fine. Made a claimed Nest, etc, no problem.
Still doesn't work from the original PC but, since it's due for a rebuild, not worth wasting too much time on it... it is just strange that it shouldn't work for no apparent reason.
My only guess would be, it may be due to some of the "unnecessary" Windows services I've disabled - has never stopped anything else from functioning.
Anyway, thanks again. I'll save the troubleshooting for the rebuilt PC, if needed... hopefully not!