After a bit of investigation into the death of my PC, I think it's just the CPU fan that's died, not the PSU. Originally, I thought none of the fans were working properly but, interestingly, it turned out to be an optical illusion - the others WERE spinning, but at a rate that made them look almost static 🙂

I figure that means the PSU is probably fine so I've just ordered a new 150mm fan. Damn next day delivery cost almost as much as the fan itself, but I need it quick..