Well I am finally back on the road after being parked up since returning from Xmas camping holiday (summer here at Xmas) Temp seems to be running nice and cool for a few days now although for the first day or so it did sit a little higher than it used to but still well in the normal range. Perhaps still a bit of air in the system. So it appears the O ring was causing at least the problem with the pump pumping dry and hopefully my long term problem of occasional overheating will be cured also. Only time will tell but thanks to all who gave suggestions and help. David