Floyd,
With regard to Mr Rielly making a claim, I would have thought there’d be some small print defining what is considered an acceptable way to ship relatively valuable items via Royal Mail and from your photos, the packaging looks as if it falls way short. Unfortunately, it looks like he cut corners in order to save a few pence.
I know it won’t help you at the moment, but I’d be staggered if they actually pay him out, assuming he’s been honest about the packaging.
I like to think that most people are thoroughly decent and that he will feel under extreme pressure to settle this with you by paying back what he owes you. His reputation is at stake here and for less than £300, he has the opportunity to rescue it.
Good luck with getting your money back