Well, I finally got the old housing off, using a combination of brute
force, pig ignorance and heat from a blow lamp. New one arrived and
fitted. OK so far ... until the same thing happened at the other end,
where the heater control valve sits on the head, at the back. The pipe
just fell off, that spigot having disintegrated, too. Oh, and having
disturbed the bottom radiator hose, that now leaks.

Hey ho. New parts in the post. Trouble with older cars - you disturb
something, and bits start failing.
--
Graeme