Hi All again,
No, getting the tail in and out is no problem - it has an external
part that has flat sections that engage easily with and adjustable
spanner. It is not the sort that has an internal hex style fitting
that need what looks like a large allen key (rad key).

One of the two still leaks - I have tried up to 30 turns of standard
PTFE tape (BS L type) and also used Fernox LS-X Leak Sealer on the
inner thread - but it still leaks. Why on earth do they make this
screw in fitting so utterly leak prone when a lot of plumbing is now
push fit. On removal this morning (after leaving it all night to cure)
I noticed that the LS-X sealant had been pushed right to the end of
the screw in valve tail and was attached to the end like a little
crown.

I note that poster Rednadnerb had a Screwfix rad that needed 50
(yeah, 50) turns of PTFE tape to prevent it leaking - mine must be in
that league.

I'll give it one last go tonight and then I'll get the plumber back in
that did the other one for us.... and guess what? that was a Screwfix
rad! That read/valves appeared to have the small valve tail as the one
I'm battling with.
Tom