My scope arrived yesterday, and after running for an hour the fan stopped and the back of the scope reached over 85°C.
When I contacted the supplier they said that they would have to get it returned, sent off for testing, etc.etc.
So I opened the scope and found the the fan was faulty, and it has 'dead spots' when it stops and won't start spinning without a flick.
I checked through my junk, found an old 40mm fan (with casing), and chopped off the case to leave only the three support pieces. These fitted when the bolts went, so I just screwed then under the edges of the bolts, and it works perfectly.
The interesting thing is that this fan is also ball-bearing, unlike the stock one. It is nice and quiet, but keeps the CPU at a pretty consistent 57°C (CPU_AMBIENT of 56°) in a 20°C room.
The importer is sending me a replacement for the stock fan, but this one is so much quieter than the stock one (when it worked) that I'm not going to swap it over.
Perhaps taking enclosed ball-bearing fans and chopping them is a better way to go? 40mm is not ideal, but as it works effectively and is so much quieter...