First, thanks for all the advice chaps.

I think it may be sorted but I'll need to wait for the weekend to shoot it to make sure.

Basically, I couldn't release the barrel shroud nut with the plastic tool even wearing a leather gauntlet! I finally got it undone with a large 8mm allen key so it had been done up far too tight at the factory and maybe that was distorting the barrel alignment.

There was no obvious damage so I reassembled it with the plastic key and using a laser bore sight i readjusted the rear sight and it ended up pretty much where I would have expected it to be. So looks like poor assembly at the factory.

As I said, I still have to shoot it to check though but looks promising.

Steve.