Yep, certainly I've found that in spades in Scotland*, as well as quite a few places in Wales. On the Black Isle and around Inverness I've been to a few chambered tombs in people's gardens and they've always been friendly and encouraging. And the pride of being the custodian shines through.

*The only Scottish exception was a Englishwoman farmer at Backhill of Drachlaw.

However, we don't know the facts here. It's possible that the owner has become too infirm to deal with visitors (I know she was elderly). If so, that's understandable, even if frustrating. What would be much less impressive is if there are new owners who don't want visitors. I'd like to think you wouldn't buy a place with something like this in the garden unless you were prepared to deal with the likely visitors, so I hope it isn't that.