Thanks for your ideas George.
I'll start off by saying that it was not the gas (lpg) that was giving me the problem because I always (as advised by the installers) start the car on petrol and only switch to gas when at operating temp.
Anyway I feel really embarrassed because the battery test carried out by the motoring organisation here was wrong-the battery was faulty and a new one has cured all the problems-wouldn't have believed it if I hadn't seen it with my own eyes. Once again thanks for everyones help and greetings from Australia.
Cheers Whitelephant