I keep hearing good things about this Jedi fellow but Hertfordshire is a long way away.
Who in South Wales has a good coaching reputation? For advanced things - gap jumps, sketchy corners etc..!
Lodge cycles in Briton Ferry - They will sort you out.
Fab reputation, and really good guys ( and girls)
As above.
For wilderness XC rides try Kim Jones at mtbtoursandtrails.
A guy from mtb-wales used to give instruction in the Maesteg region, and while I never used him for myself, he did instruct one of my sons, and I thought he was excellent.
Ambrose, seeing as he is more out in your direction, do you have any contact with Andrew?
Looking at the suggestions above, mind, I doubt that the person I am thinking of would be your kind of instructor. Perhaps [url= http://www.forestfreeride.co.uk/ ]Forest Freeride[/url] if you can get to mid-Wales?
I spent a weekend with Stuart, and I thought it was brilliant. If you look in my history, you will find a review I wrote of my time there.