Locate evaporator thing under the bonnet, check that vacuum line is ok. Then change all LPG filters. If still have some issues and want to sort them out yourself you should consider to buy a diagnostic cable for your lpg system. It could be cheap, it could be not. Depending on age and mileage distributor could need rebuilding(fairly simple). Injectors could need rebuilding or change.
Usually injectors connected to lpg Ecu and then from ecu to petrol injectors. Check those connections.
And so on.
Start with pictures of the system, then phone those guys in Birmingham to find out if there was any extras.
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