If your willing to travel a bit there's, creech st Michael in Taunton, small site, big half pipe and little woods to drive up and down, few steep slopes, row of humps, its ok for a few hours play, really not sure when it's open!
Bristol has Dundry, big site but they haven't made use of much of it! And only go if we've had dry weather as it's got a few natural springs, so very muddy you will get stuck! Open first Sunday of the month? I think..
Wiltshire has foxham, ok site but fairly small has a few fun bits to play in

open third Sunday of the month again I think??...
Or if your willing to travel there's muddy bottom down Southampton way, good site quite big and there always improving bits of it.
Think that's open first Sunday of the month, there doing a camping weekend too in September I might go to.
Other than that there's avon dassett in Stratford Warwickshire again quite a way to travel and not sure when open, but I really enjoyed it there in the disco, loads of descents/climbs mud!
Can't think of any more off the top of my head. Wish there was more though!!