Just an update if anyone has same problem ;
Have changed the thermostat and all seems fine ,
Thinking about the problem , If you had a head gasket blowing the pressure in the system would keep on building up and up until something blew eg,resevoir cap / pipe ect ,this did not happen on mine all you could see was that the water level had dropped but when you removed the cap the water returned to normal,What must have been happening was that the water was being pumped through the engine and the heater that is why the temp gauge jumped up very quickly to normal running temp in effect the engine was getting very hot. (how it did not over heat i do not know as i have done probably 2000 miles in this condition)
The question is did the water boil turn to steam causing an air lock which caused the pump to suck water out of resevoir ,not sure if boiling water in water causes steam,
Anyway she runs fine and heater works as it should and temp rises gradualy
Hope this can help someone in need.