DanW, riding is tolerated quite well. The black canon have an area which they can build in with agreement with the longleat estate (which you have to be a member to ride or be shown round by a member). The rest of the woods is non official with lots of singletrack. Cant think of a trail that has been destroyed by the estate, apart from logging!
here I am mincing around,
[video]http://www.youtube.com/watch?v=fY16LyYSJmg[/video]
[video]http://www.youtube.com/watch?v=I5kdCNmfq-E[/video]
EDIT, there was an issue a couple of years ago when some people built some dirt jumps on an old fort which is an SSSI in the woods on the frome side of longleat. but that was obviously just stupid and once found out was not accepted well by the estate or the authorities. thankfully this didn’t affect other riding around the estate.