1995 LPG/Petrol conversion not starting when warm

we have a 1995 Ford escort which has been converted to also run on LPG. When the engine is cold it starts without any problems at all either on petrol or on gas. However, once the engine is warm, if we stop and switch the engine off it will not start again either on petrol or on LPG unless we bump start it!! The garage is saying the whole system needs replacing. Any advice is most welcome