Does it run correctly on petrol only or does it put the EML light on?
If you've never had LPG before you may not know how it works. The car's ECU still sends signals to the petrol injectors and the car still thinks it's running on petrol, but in reality, the gas ECU intercepts those signals, modifies them and sends them to the gas injectors instead, so it has to be running perfectly on petrol first before you can even begin to look at the gas system.
If you've got an MOT to pass soon, I would suggest that now, when the EML is already on, is perhaps not the best time to start learning something new. Take it to a professional with a good reputation to get this immediate problem sorted and then you can learn at your leisure when it's not critical and you have more time.
One thing you perhaps could do is that when it goes for MOT you can choose to present it on petrol or gas, so if you took the fuse out of the gas system and presented it on petrol alone (as long as the EML doesn't come on on petrol) you may get it through.