OK, so more progress has occurred in the last 3 days than in the 6 months this has been running...
I have now tidied some the cabling and built the unit back into it's original case.
I was really lucky and found a 12v switched (powered on and off with the projector fans) supply coming direct from the power supply. This now runs an old Athlon 64 (CPU) cooling fan attached to the old Athlon 64 heat-sink, which runs the LED at a comfortable temperature (I am not sure what the temperature is, but it doesn't burn my thumb, so it's within tolerable limits).
After it was all back together, I decided the gaping hole in the side of the unit was a likely cause of problems, so I found the scrap bit of plastic I cut out originally, drilled it full of holes and stuck it back in. I can't tell you how much it pleased me I hadn't thrown it away.
Attaching the LED turned out to be as easy as 2 self tappers into the heat-sink. Not my favoured solution, but it's not going anywhere.
Anyway, time for a soak test.
See the pictures for the near finished article.