Thanks. I've put the old PSU back in and everything works fine, so the new one going bang thankfully does not seem to have fried my computer.

I did worry I'd connected things wrongly, but I've just plugged the connectors in the same way with the two PSUs, so it's not clear how I can successfully install the old PSU twice and yet blow up the new one. (I suppose the new one might need connecting in a different way, but this wasn't clear from the instructions or physical appearance of the thing).

BTW, the new PSU is a jeantech 500W - it's the only brand PCWorld sell, although I bought the most powerful (for the bigger...err...bang )

I think I'll try to get a refund from PCWorld and make an appointment with them to install a new one.