I should have said I am running XP throughout at the moment although I sometimes have some Me and W98SE machines on the network.

I note that there does seem to be quite a high failure rate being experienced in general. Oddly my D-Link seems to ahve resurrected itself so I am happy at the moment.

TO deal with some of the other suggestions: -

Yes, I have had an electrician in (and he was computer savvy and knew about ethernet over power too) and he OK'd my mains all over the house.

Interestingly the ethernet over power works less well via the separate switch than straight into the modem-router-switch and that may point again to a problem with those devices.

Simon, do those polyswitches fail if overused? That sounds very like my issues.