Just to update on the changes for getting a disabled pass at Chessington's, we went today and had no problem with getting a pass. I had the lad's blue badge and a rather general NHS review letter which quoted him as having profound autism. So it seems that any offical letter which mentions the dx should do.
We had a really nice day and he really enjoyed himself, with lovely wether too. I was personally gratefull that we didn't do the local option of a zoo built on a massive hill!