SupaMac using their clients cars for personal use

SupaMac, also known as Master Mac since recent name change, uses their clients vehicles which they were supposed to be fixing/servicing, to collect parts for other vehicles. This was not agreed upon and they use your petrol. Then if something happens to your vehicle whilst doing collections they lie to you about what happened then they make the mechanic who was driving pay for the damage. They do not ask permission for the collections and the drive for collections isn't part of the test drive. This is complete personal use of your car.