If it were me, I'd be chasing Scotsure to see when they thought they gave CS authority to strip the box, and would want the recording of that conversation to prove it.
My guess (and it is a best logical guess), is they somehow gave Mitz authority to do the strip down on his original call, but not authority to do any more. They are using that fact as a get out clause to say that work had commenced, and now your claim is invalid. Very poor service if you ask me. If work had started, then fair enough, reject your claim, but as the car hadnt even been to the garage for work, that is very very poor behaviour from Scotsure.
As I say, I would be chasing up for a recording or the dialogue of that call to prove it was given.