Hi Sean,
Know what you mean about Bovington, very tempting though, If I had to get locked in anywhere for an evening I might just choose there :-) :-) :-) :-) :-) .
There was a post not too long ago, someone, sorry can't remember who, is scratching a Churchill on the beach at Dieppe, and doing a really good job of it, I think it showed the top hatches open so you might get a reference there if you can track the post.
Also check out Cromwells, sorry don't have their web address at the moment - they do lots of conversions and you might get a useful picture there.
I can't remember if I got any shots of the hatches when I was last down at Bovington but Vinnie posted the pictures some time ago, there maybe something there.
Cheers
Al