When I got the bike there was no gauze filter in the sump and the end of the pipe was damaged. It had some of the end missing and what was left had a split in it. I fitted a mesh and new gaskets. However, as i said now that I've got the engine on the bench I can see that the ball bearing doesn't move and it looks like a piece of the spring has got past it.
There was never enough pipe to get hold of to pull it out and my attempts the remove it have certainly added to the damage and the end of the pipe where it was split has broken off. So it definitely has to be replaced now
Following your earlier reply I checked the other non-return valve and it appeared to be ok.
I think that the presence of the thread on the inside of the valve means that someone has either had this out or attempted to get it out in the past.