In the end it turned out to be a faulty earth wire to the fuel shut off solenoid, which made the fuel solenoid drop out whenever the fan reversed (drawing more current).... It still did it after I cleaned the plug contacts and the connectors of the earth supply on the body near the bottom of the battery (not a brilliant spot to get at).
I first unplugged the solenoid and connected it to another battery to see if that would fix the problem and to confirm that the solenoid wasn't faulty: and it worked! I then piggybacked another earth wire to the black one on the solenoid (on the solenoid side of the connecting plug) to a good earth on the body which fixed it. Only took 3 days of stuffing around !!!