Harvey, Petr, Phil,
I now have the black machine back after the new transfer valve has been fitted (3 hours labout by the way). Although it is early days yet, it does appear to have completely fixed the problem. I have only driven it for about 30k, but I have done a number of sharp turns including some full lock turns in the company car park. There is no trace of the juddering/binding at all.
So, I am delighted.
Many thanks to everyone who has given me advice, and, in particular to Harvey and Phil, without whom I would probably have ended up having the whole gearbox rebuilt, possibly without solving the problem.