According to James Dyer, in his book 'Discovering Prehistoric England', the stone coffin was rescued from a nearby barrow which was trashed in the 19thC, I think (I'm working from memory here as the book's at Moth's house). I couldn't make sense of the lichen-covered minilith at all. Perhaps it, too, was rescued from loss and trashing at the same time as the cist? God, I love stone cists!
If you haven't got 'Discovering Prehistoric England', get it Steve, it's really rather splendid.
http://www.amazon.co.uk/exec/obidos/ASIN/0747805075/qid=1085344364/sr=1-1/ref=sr_1_10_1/202-9710664-8188603
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