I finally got around to having a good look at this, the heatsink was on correctly, I've changed the heatsink to some of that fancy Artic Silver 5, put the old noisy fan back on, and it was, if anything worse. Heatsink gets warm, runs OK with a big fan pointing right at it, lasts about 10 minutes with no big (external domestic) fan on it.

I think I'm going to give up on this thing :(