Bongo Fury

Problem still seems to be their I checked system for leaks and cant find anything so dont think its overheating due to low water level as I check this regularly just had thermostat changed as a precaution. It does look like the fan comes on as soon as engine is started though.I have had both AA and a garage test for head gasket and they both said it seems ok.
ANY IDEAS WELCOME.

Not sure what was previously said but is air con definitely off?

the only reasons the fans come on are its hot or the battery voltage is low, this can happen especially when or after starting, starting drops the battery, then the glow plugs come on and flatten it more. while starting if the battery drops below 9.6 volts during cranking its knackered.
this could mean that the battery is knackered, or a glow plug is faulty so going to earth when switched on, or the glow plug switching system is faulty.
or the fan switch is faulty.

i would put a meter on the battery during starting and see what is happening, then try putting a meter on the glow plug system to see what is happening, then take each glow plug out of the system to isolate each plug in turn. you can measure the resistance of each plug whilst its in the head, the manual has info on how to do this. and how to test the fan switch.

checking a battery with a good multimeter.

https://www.youtube.com/watch?v=tsJUuLu1cw0

Thanks for help its now looking more and more like the radiator could be partially blocked so will have ago this weekend.