The tomb you're thinking of is Barclodiad-y-Gawres. Bloody ace. We stayed in a cottage opposite it last August.

See my weblog about it here http://www.themodernantiquarian.com/user/1761/weblog/0/50412 and treaclechops' here: http://www.themodernantiquarian.com/user/1765/weblog/0/50520

J
xx