Well, I replaced my fan and I'm not dead or injured, so thanks for the
advice everyone :-)

Just in case I'll be doing this again though.....
Would shorting one of the positive leads to ground on the +5, 3.3 or 12v
pins through a resistor guarantee an empty pair of caps on the offchance
that the PSU was so cheap that they didn't put bleeder resistors on?

Ewan