Thanks for the replies so far :)
Just to clear up an misunderstanding – the PSU hasn’t been moved out of one computer into another. I installed it into my mothers computer while I was at work, and tested it there, and it worked fine and was turned on for a good couple of hours no issues. Took the computer back to her house, and the PSU blew as soon as I turned on the socket and extension cable. Didn’t even turn on the computer.
I’ve ordered another PSU – they are only £20 on eBay. Dell won’t be interested. The computer is about eight years old, and she only uses it to print letters and photos.
I’ll be testing it again in her PC, but not in her house. If that works ok, then I’ll be trying again in her house with a different power cable, and plugged directly into a different socket.