Just heard from my man (the chap in my group who collates all this type of info - he may be on here in a bit to clarify), and this info is in the official Conditions of Sale pamphlet.
In 2005, it said that any grandstand ticket was valid in A,B,C,D parts of 34 (ie that's all the stands above the pits). The letters refer to the staircases at the back. It also said that access was allowed to the roof of the old ACO tower.
In 2006, it again refered to A,B,C,D parts of 34, but of course, no mention of the tower, as it had been pulled down, but added you could gain access to the 'Pit Lounges'. We tried, last year, to find these, but couldn't.
This years pamphlet only mentions the Pit Lounges, so maybe that facility has been withdrawn.
I've only got a pdf file of the pamphlet, and if I can work out how, I'll post it here.
Edit: He's been looking on ten tenths, and it says on there that access IS allowed! Perhaps this needs to be confirmed (or otherwise) with the ACO. Termie - another point to add to your list
Also find out where the hell the 'Pit Lounges' are!Del