Thank you DD and Rob for your useful info.
I do have a replacement, which is the reason for the removal.
The new one is alloy and does have a hex inside the tube.
The original has the external flats. So I guess that makes it an all plastic one.
Carl very sensibly did not give it the big heave ho, as his experience told him that it might bust, and there was no particular pressing need to get it off at the time.
Just need to find a suitable box spanner, or possibly an oil filter gripper and hope for the best...Good to know about the "shelf". If it does bust at least I can push some plasticene or somat down the 'ole to stop bits going in when I chop the rest out.
Why on earth would anyone bond it in? It can't turn with a pipe clipped to it.
Anyone know of a proper tool? (for the breather....
)