there was an article about this in horse and hound a couple of weeks ago, (the one when lucina green rode springalong) theres smallwood http://www.smallwoodliverycentre.co.uk/ which I like alot, and you can also use the xc course at skipton after the event has run in June.
If you book onto the www.eventingworldwide.com clinics with Bill Levitt then you can use Kelsall Hill. Its a double bonus, he's a good trainer, and there are some interesting fences at Kelsall. The next one is this coming Sunday (with others July 15th, August 18th and October 7th), call them if it sounds appealing, I'm not sure of the cost... 01400 250472 or 07973 745267
Other than that (assuming Somerford is convenient) then it has to be Smallwood as already mentioned.