Formula 1st Tyre-exhaust & Auto Centre Ltd

Booked my car in for a service on 25th June. Had been using the garage for years without incident, so was expecting good things as usual. When I collected the car, the service light hadn't been reset, and the service book hadn't been stamped. I was going away on holiday the next day, so checked the oil - black and dirty, not new oil that had done just six miles. I have a dashcam fitted to my car that records video and audio, it's very obvious when you are sat in the car... it however appears the "mechanic" didn't notice it when he was "servicing" my car, or when he took his friends for a ride in my car to buy his lunch on Worle High Street. I've tried contacting the garage twice about this without any response, which is a shame. It seems the service wasn't completed, and my car was used for staff to go shopping. I'm rather hoping this is one rogue member of staff, but no response from the garage is strange, and almost appears an admission of guilt