Cost of fridge engineer

Just over one year ago we bought a Unicorn Pamplona which was three years old. It had spent almost all of its life doing nothing so was absolutely like new except that we have had a couple of minor problems because of that lack of use.

For some reason and I really don't know what, I thought I would put the Dometic fridge/freezer onto the gas powered option It would't work. It works well on 240Volt and from the car. I have no sparking and no gas at the burner. I've done the obvious common sense checks like fuse and stuff, but nothing.

To be honest, I don't really need to run the fridge on gas but it irritates me that it doesn't. I've looked around the internet and can't find any idea of how much an engineer might charge. This is important for my cost/benefit choice.

Would someone please give me an idea of how much these guys charge please?