I thought about that, but it seems like the stonework is very finely laid with no obvious cracks. As I recall computer room floors could be lifted with special suction cups. This wouldn't work with stone. This guy's blog makes it sound like there might be a hidden entrance. I believe Nancy is supposed to be interred in the same tomb so there must be a way to reopen it at a later date. One question is whether the tomb is beneath the stone circle or in the wall behind.
Well it sounds like if all else fails, someone can ask the blog guy.