Provided that the parts were fitted correctly and the correct application was adhered to the supplier will be liable and not the workshop. If sub-standard parts or lubricants were used, then the responsibility shall rest with the workshop to compensate the customer. Bear in mind however that the Consumer Protection Act allows the consumer to pursue any party in the supply chain for compensation, this includes the workshop. Therefore it is vital to ensure that you make use of a reputable parts supplier who will stand behind their products and provide compensation when the product is faulty.