There's actually a huge amount on the relative dating of long barrows, with Bayesian analysis being used to greatly refine chronologies. For the British Neolithic, it's been used most comprehensively for causewayed enclosures but also touches on long barrows and other broadly contemporary monuments. You'll need a decent bank balance or a handy university library to read this stuff:
The main publication is
A Whittle, F Healy, A Bayliss (2011) Gathering Time: Dating the Early Neolithic Enclosures of Southern Britain and Ireland (Oxbow Books)
but the earlier work on long barrows was published in the Cambridge Archaeological Journal - volume 17 (part 1) included a paper by Alasdair Whittle and Alex Bayliss called "The Times of Their Lives: from chronological precision to kinds of history and change" (pages 21-28), while the same volume came with a supplementary volume with 5 detailed case studies of individual barrows called "Histories of the dead: building chronologies for five southern British long barrows", edited by Alasdair Whittle and Alex bayliss.
Also worth reading is Frances Healy's summary of the dating evidence for the southeast, available for free download at
On long barrows generally, in addition to Dave Field and Tim Darvill's recent books, also recommended is Martin Smith & Megan Brickley's (2009) 'People of the Long Barrows: Death and Burial in the Earlier Neolithic' (Tempus).