My brand new Mercedes-Benz caught fire. It broke down and I didn't get any warning signs on the dashboard so I called Mercedes roadside assistance and they sent the RAC to check my car. After two hours of checking he couldn't find a fault than he drove the car and car was fine again so he said take it to Mercedes tomorrow and he left. I was 40 miles from home so got back on the motorway after roughly 15 miles it started jerking again so I came off the motorway and pulled over. I opened the bonnet and there was fire underneath the engine - by the time the fire brigade came 50% of the was on fire. Who is liable?