To update, I ended up fitting an Antec 380W PSU
http://www.antec.com/Believe_it/prod...==&id=MjIyNA==
It was slightly narrower than the standard Dell PSU at 86 instead of 97mm. And only 2 of the four mounting screw holes lined up on the Dell case, but hey it fitted and the cables had plenty of length to get to the remote parts of the case. I suspect I could have found a cheaper generic or unbadged PSU if I'd looked around. Quite a few though had fans mounted on the top or bottom, rather than the end, and that wouldn't have worked with the way the PSU had to fit the case.