Geoff, how old is the fridge? and what model? there should be a plate just inside the door, with an RM number on it.
If it is an older Dometic/Electrolux unit, it may be that when you push the gas valve knob in, it is not going in far enough, and needs a small packing piece inside the knob to give it the extra "push".
Other than that, it may be one of several things, firstly that it just needs a good service, maybe a partially blocked jet, maybe the thermocouple is malfunfunctioning, or it is a faulty gas valve.
As it involves taking the fridge out to check and service, it must be done by a qualified LPG service engineer as it involves breaking and making gas couplings which make it fall under the Gas regulations.