Check out mroc.co.uk, I'm a member and they do a couple of play and plays a year, but these are properly laid out courses with "experts" (Read: people in knackered Series 1's like me, who enjoy telling people what they've done wrong, we'll always be nice about it mind

and we're always happy to ride shotgun a talk you through it) and contain "easy" routes. They also do TYRO's (Try your vehicle offroad) which are greenlaning level difficultly with expects on hand to help out.
O/T they also do RTV's are for road taxed vehicles, but to be competitive you've gotta not care about the body work, have a good few inches lift and be running mud terrain tyres, CCV's are most extreme again with most being 3.9/3.5 V8's with full cages. Another step up again is the Comp Safari, which is like the CCV, but longer, and against the clock. Theres one tomorrow not far from Stratford if you wanna come down and spectate (Always welcome

)
An 02 plate Rangie came to our last play day, got stuck a bit, but they seemed to enjoy themselves

(Its the road-biased tyres more than anything) The Series is mine btw

(The rest of the photos on my flickr are all MROC RTV's)
I must warn you though, the last guy to show up to our play day in a Disco 3 came back to the RTV the next week in a £2k fleebay special to play more seriously
Also check out the Land Rover magazines, most usually do a greenlane of the month type thing. I would however advise finding a friend to greenlaning with - If you get stuck in that Disco, you're on road tyres, without a winch or a ground anchor and inaccessible to the AA ie up **** creek without a paddle.
If you're doing the Experience (Highly recommended) try the Eastnor castle in Herefordshire site, its bigger and better than the Solihull plant IMO
To summarise all that waffle:
Most play days = Bad for a D4 ~ £20
Structured, club run play days = Great fun, will learn a lot if he asks the marshalls ~£20 IIRC
Greenlaning = Easy and fun, BUT TAKE A FRIEND IN ANOTHER 4X4 ~FREE

TYROs = As simple as greenlaning, but others there to help/advise ~Not sure tbh
RTV's CCV's and Comps = Not suitable for a D4 at all, but great to watch

~£20 to compete, free to watch
Next TYRO is 5th of April at Eastnor Castle (The home of Land Rover)
Next MROC play day is the 16th of May, I should be there, Uni work-load permitting
