... the stones were dropped into water.
http://www.themodernantiquarian.com/image.php?image_id=4886
The hearth for heating the stones is at the back and the trough that held the water is at the front ... full of water.
lots of cracked stones were found thrown around the outside of the area in all directions, which would indicate that there was no covering over the site.
Not many burnt mounds have been excavated in the UK (I think), but quite a few have in Ireland where they're called Faluchta Fai (or something similar) which means deer cooker (or something).
It could have been a