boiler not heating water?

have a sabre he boiler in is fited in a park home i have just bought it so dont know a lot about it it has been un used for about 8 months the hot water is not heating up right some times it will come look warm sometimes it will not work at all the boiler seams to fire up when you turn the tap on but doesent heet up it is runing off 2 gas bottels and auto swiche over regulater any ideas what the problim might be