Yesterday I borrowed a library book which I know I am going to enjoy. Its called 'Earthen Long Barrows' by David Field and is probably well known to people on TMA. I fell open at a page headed Wooden Mortuary Structures and rather than try to interpret it, I have quoted the passage below.

Quote:
In the literature, the terms burial chamber, mortuary chamber and mortuary enclosure are often used interchangeably. In some cases these are hardly chambers and some may not have been roofed. To avoid confusion the term mortuary structure is used here for an enclosed box-like feature that invariably contains skeletal remains often placed at the easternmost limit of the long barrow. Mortuary enclosures are larger area and may not contain skeletal material. Thus the 'mortuary enclosure' referred to in the Nutbane report, is in fact a small almost square structure 7x5.5m built of wooden uprights and horizontally laid logs. It had three posts in the north, five on the south and three in the east and as the horizontals continued beyond the verticals it was evidently not a joined structure. It was seen as linked to phase two of the forecourt structure, but was evidently later than the primary burials, although a further, potentially contemporary burial was later inserted.
End Quote:

Taken somewhat out of context from a chapter titled The Structure of Long Barrows, I hope it makes sense. I'm not sure (yet) what the author means by 'potentially contemporary'.