Exactly as per Cliff S1 comments I totally agree with his point of view.

I suggest you speak with a solicitor and get advice in relation to the situation your in. I suggest you leave the car in the garage to avoid any confusion with who is liable for the damage. I can see this unfolding already."The car was fine when it left this premises"

Get some photographs, which is evidence if the case ever went to court. At the end of the day they are liable for any damage caused to your vehicle whilst in their possession. They require PI/ Insurance cover to operate a business. I wouldn't assume the cause of the fire I would try to establish this. I suggest you contact the Police and have the accident on record.

Best of luck buddy.