Steve,
Hope you and Iona are well.
Having ridden with you last year Treads MTB, I was the good looking Welsh bloke on a 4" XC bike
I suggest Thetford for some basic single track skills I know the TIMBER chaps have a regular group ride Forum Link to find some of the more technical singletrack.
Then over to Woburn for some trails with gravity assistance Cyclone are a great bunch of chaps having ridden with them before
If a little more confidence is required I have heard only good things about UK Bike Skills who use Woburn as a venue to coach riders. Been off my bike for a few months considering a session myself
Not been to Chick Sands for a while but if the website reflects the bike park this may also provide a good venue to hone some skill.
A little further a field you could suggest Llandegla, near Chester the Red/ Black trails will give a good indication of skills level, there are also skills days held there which will provide a good bench mark.
And being Welsh can not forget
Glyncorrwg Mountain Bike Centre , Whites Levels, The Wall and W-2-W, July trails will be another bench mark
Coed y Brenin in the North should provide a good bench mark for ability, look towards Tarw, MBR, Dragon's Back and The Beast of Brenin trails
Hope this helps
Mark