I havn't tried for Julieberries Grave yet, I have a couple of other things to see in Kent (I think) so I waited for a final catchup.

The coffin stone in in the field immediately to the west of the one with the countless stones in. Basically leave the countless stones following the pylons across the road. Work your way through the hedge somehow and the coffin stone is in a small 'island' in a vast sea of crops about 200m west from the hedge you just crawled through (there are entrances)...

Enjoy!