I have tested the resistance between the neutral and ground terminal of my sauna heater, when disconnected from mains and get a reading of some 30 M ohms. The trip cuts out only occasionally . When I remove the earth wire from the sauna heater there is no more tripping but I am not happy to leave I like that. Nothing else on the circuit trips out.

The resistance of neutral to ground should be almost "0"...
And you are right, "NEVER" disconnect the equipment ground...
If the GFCI breaker is tripping you may have a potential hazard that should
Be fixed or you may need a new GFCI. Older model GFCI breakers from the 90's
Did have some problems...
Consider hiring a good electrician.

Thanks for the reply.I am fitting a new double pole GFCI tomorrow the original ones are all early ninetys. Surely earth neutral resistance of 30 meg ohms should be negligible . I think the trips are too sensitive for sauna heaters and water heaters etc. I am hoping that a new one with nothing else on the circuit will improve the situation .

Merv,
At your main electrical service the neutral is bonded to the ground... therefore there should be very "low" resistance between them (theoretically zero)... if you really have 30M ohms you have a dangerous problem that "needs" to be corrected!

Merv,
At your main electrical service the neutral is bonded to the ground...therefore there should be very "low" resistance between them (theoretically zero)....if you really have 30M ohms you have a dangerous problem that "needs" to be corrected!
Thanks, the main electrical service neutral is bonded to ground as you say
I meant the high resistance was present between neutral and ground on the sauna heater when completely disconnected from mains . This would be correct. I fitted a new double pole gfi today. Tested and working properly. Time will tell if there is a real problem with the sauna heater it's self. Interestingly .I could not see where the earth and neutral were connected behind the main electrical service board. Are they tied inside the electric meter? Which of course is tamper proof .

Typically yes... inside the meter, then at the "first" overcurrent device.
If the neutral bar in the panel is not physically connected to ground, the ground wire, grounding electrode conductor and the panel enclosure, it should be.