I have already removed the small grub screw but the barrel refuses to budge. Is there a way, maybe from inside, of levering it without damaging the outside of the door?

Have you pulled on the outside handle?

Should that help or hinder?

Should that help or hinder?

Help. Assuming you have removed the retaining screws for the barrel, hold the handle open so it's not pushing on the lock escutcheon. You should then be able to push the escutcheon towards the handle and out.

You have loosened both retaining screw?