I was able to ask the seller who made it:

Any idea who it was who made it, its really nice piece of work.

It came from the workshop of the small family business of Sorrell and Son. Whilst I haven't shot this one myself, my son has one of these and he says it handles and shoots really well, with all the recoil directed straight back and no muzzle lift at all. There is a 16.7" barrel version as well which is really cool.