Garage cost us £2500 as insurance comp won't pay.

allegedly

Thanks for your reply SouthUKMan,
Yes, that's the route we think we need to go down now.
We are gathering as much 'evidence' as we can.
The initial roadside mechanics report and the phone transcripts hopefully too.
If we can get the calls where the insurers told the garage and my son in law it was up to him to authorise the exploratory engine strip, that's okay too as that is what happened.
The obvious missing one should be the garage calling the insurers, outlining the damage found and asking for permission to carry out the work. This should prove it never took place and the garage acted on it's own.
Then the one where the garage sent details of the damage to the insurers long AFTER they'd actually carried out the work.
Once we've got these and established a 'time line' of events, we will be contacting a local solicitor, unless anyone can recommend one who specialises in this sort of case.
Thanks to everyone for their replies, fingers crossed it turns out the law is not blind in this case and see's the error of the garage owners actions.
Thanks
Rob