When I sorted out the Wgn Sqn mission at Delta Papa I contacted this guy...
Peter Farnfield
pfarnfield@rutland.co.uk
01483 542 224
07971 402 339
His name is still on the 'Contacts' link here...
http://www.dunsfoldpark.com/index2.html ...so I guess he's still around.
DP may let you on with 40 cars, but you'll need to be marshalled which is going to put the cost up. A full day 'as a photo shoot venue' was £500. Add in marshalling (if they provide it) and it could get a bit expensive.
We used the end of the disused runway for the best shots. You would probably get 40 cars on it, but could be tight. I would suggest a mission ‘reccy’ beforehand.
Contact Peter... as he may surprise you.