hetty peglar's tump - top site!! and I can agree with you about the feeling of unease you mentioned in your post. The first time I visited I didn't have a torch and severely lacked the courage to go into the pitch black side chambers. Nice and cosy though in another way.
They've put some gravelly stuff down since? it's not so muddy and a bit more heritaged-up now, I quite liked it with the mud.

glad I didn't see any spiders especially LARGE ones (UGH I can't help it)