Top reply, Pitcrew!
 
 Just to add re the last point, I seem to recall that when Celeste got the bike it was running really rich.............possibly to disguise a long-standing problem??
 
 One other thing, from experience if a headgasket is gone, it will pressurize the cooling system within half a minute - ie well before the thermostat should open.
 
 Cheers
 Paul