On my GG 270 the bike will run for 10 mins then the thermostat pops out! Thinking the water pump may have packed up and that is pressurising when hot? Any other ideas? Thanks 

The pressure relief valve may be blocked and that could be the weakest hose connection

Head gasket pressurising the cooling side

I take it that its the "in line" version. If cooling circuit overheats (could be a number of reasons),  the first thing to do is check the fan cuts in by shorting out the two wires on the 'stat.. Fan operation  can be checked with a 12 v battery. Fan is also controlled via a regulator ( the small black box). If this isn't earthed correctly,  regulator cooks itself & fan wont run. Replace the thermostat rubber sealing ring & retaining clip as it will very llkely be u/s.  They split / distort quite easily. Only costs a couple of quid to replace. 

Also worth checking water pump. Water pump shaft is made of low grade steel & wears at the lipseal contact point, resulting in water ingress & milky gearbox oil.. This is usually the first indication of water pump defect.  Water pump overhaul kit easy to do, but not that cheap.     

23 hours ago, pjw123 said:

I take it that its the "in line" version. If cooling circuit overheats (could be a number of reasons),  the first thing to do is check the fan cuts in by shorting out the two wires on the 'stat.. Fan operation  can be checked with a 12 v battery. Fan is also controlled via a regulator ( the small black box). If this isn't earthed correctly,  regulator cooks itself & fan wont run. Replace the thermostat rubber sealing ring & retaining clip as it will very llkely be u/s.  They split / distort quite easily. Only costs a couple of quid to replace. 

Also worth checking water pump. Water pump shaft is made of low grade steel & wears at the lipseal contact point, resulting in water ingress & milky gearbox oil.. This is usually the first indication of water pump defect.  Water pump overhaul kit easy to do, but not that cheap.     

Thanks it could possibly be the regulator as fan does work when connected to a battery but not when bike is running and not when thermostat wires connected, also the thermostat rubber and clip are probably worn and need replacing too

Regulator about 25 quid . Try your local dealer first, as it is a common problem, & he will no doubt help you out. Otherwise its from GG UK, SplatShop TBB, etc, etc  Make sure its correctly earthed when you fit.  Wiring arrangement on later version of regulator is slightly different to the earlier type, so make sure you have the correct details of how to wire it up. If your gearbox oil is OK, the water pump shaft seal should be OK, but for a bike of that age, a bit of preventative maintenance won't go amiss. Check out impeller, & bit of marine grease on the shaft /seal.

Regulator / Clip / seal will cost £30 if you do it yourself. Water pump overhaul about £70, again if you do it yourself. No special tools required as I recall..