After some head scratching I have discovered the cause of the issue with the fast speed radiator fan being constantly on. The car was fitted with a stainless steel spare wheel tray, it has five holes drilled in it to prevent it filling with water. The deluge we had the night of the MESS meeting ended up finding its way to this tray, and I assume a decent amount of it went through the centre drain hole in it. This hole is directly above the live feed wire to the fan motor, and the water caused a short which left the motor running.

The wiring is now dry, there is no short and the motor runs when it is supposed to - the drain hole has been blocked.

'The MESS' would like to take this opportunity to thank Glyn Howell for this very handy tip. Thanks Glyn.