Agggggh now i've done a bit more reading up on the longbarrow as to where i got the original map reference from i did a bit of searching online and found the following links:
http://ads.ahds.ac.uk/catalogue/adsdata/arch-302-1/dissemination/pdf/disk8/fwp74/r4edfyf.pdf
http://ads.ahds.ac.uk/catalogue/adsdata/arch-302-1/dissemination/pdf/disk8/fwp66/fwp66b.pdf
These refer to the barrow and on reading these it mentions the confusion on the position of the barrow.
Also, reading these and the posts on the MA about the barrow do you think what you found is the original site of the barrow (fingers crossed it is as that would be excellent!) or where the stones of the chamber + facade had been moved to?