I'll mail you a map around Longleat – hopefully you'll be able to make something out of that – or get up there tonight if you're available; Black Canon Collective do a ride from the pub in East Woodlands at 7pm (I think they still do over the summer anyway)
Edit: Just looked – they're out on Tuesdays now.
The MoD came down a bit hard on us after Set2Rise – it is officially out of bounds to all – dog walkers, horses, moto-x, downhillers and us on MTB's; they are apparently going to be fencing off a large section.