Rob Frances lent me his removal tool and it removed the previously tapped in (and rusty) ferrules without any problem at all.
Simply screw the central bolt into the plug hole (his is 18mm dia to suit 18mm plugs) which holds a swivel nut (with spigots to engage with the ferrules) against the ferrules. then unscrew the nut. Simple!
I am going to get one made by a local engineer. If anyone wants one, let me know. I should have a price tomorrow.