we never make mistakes with our repairs

I had an accident in june 2013, and february 2015 I noticed grey marks appear on the bonnet of my car, which is what Classic Coachworks repaired. I took the car back to them on the 13/03/15 to enquire about the guarantee on the paintwork. I advised them that no one else did any work on my car. After 3 wks, they got Ppg to do a report on my car. Today I get the report back with the conclusion that additional paintwork was done on the car after they worked on it, although I sd there wasnt, so how do I prove this? I find out today that they've been using Ppg since 2013, so how do I even know that that was their product used on my car? How do I know that they didnt spray my bonnet twice to result in the think paint, as I wasnt present when the car was sprayed. Is this what my future repairs with Discovery will be like? U pay a premium, u have an accident, they send u to a ***** panelshop, who catches on **** and then acts like ur to blame for their bad repair. Whats upsetting is that Discovery now expects me to do the runaround to sort their mess. So if I cannot trust my insurer to help me when I have a problem after a repair, should I go somewhere else?