I think I have found the problem.
The coolant fan control unit was putting up too much of a fight, so after some more reading I hooked the battery back up to see if any codes were stored. The usual one for this car was there, 17911 alternator output implausible which I have never got rid of and 16917 G65 high Input PO533. I couldn't get any HVAC codes as the unit was removed and the fuse would blow anyway.
So, I left the coolant fan alone and went on a search for the G65 aircon sensor, the other sensor that is in the circuit. It is accessible from under the right hand front wing, no need to remove the undertray.
I tested the resistance to earth from the fuse output, showing 233 ohms. This looks promising. Removed the connectors to the doors, and it then showed no continuity. My conclusion was that the 233ohm normal route to earth is through the electric windows and the G65 sensor had gone short circuit. I tested the pins on the connector to G65 and none of them was showing a direct connection to earth, so it didn't look like there was a loom issue either.
Reconnected everything, apart from the G65 sensor, and everything is working, no fuses blowing.
A rescan and the engine error codes have cleared, did group 8 the HVAC codes and P01274 had popped up, V71 air flap positioning motor. I cleared that and it has not returned. I'll have to read up on the climatronic diagnosis / reset procedures and do a rescan some oither time
Looks like I need to get a G65 sensor, but I now have heat and a fan, that's good enough for the moment.