I have recently purchased 3239 and have noticed that the sail is 6 to 8 inches short of the top of the mast(please see linked photo). Obviousely I have the older style 21ft 6 mast with a new stly sail (hence the longer foot). My question is should I lop 6 inches off the mast (hogg stepped) or just leave the sail 6 inches short of the top?

I know almost nothing about the subtleties of Merlin rule history, but that looks more like a shrunken luff rope (it can happen) than a mismatch between mast and sail.

No, that's a 21ft sail on a 21ft6 mast.  I'd leave it as is.  You won't gain anything from lopping a few inches off the top and you may at some point want a sail to fit your taller mast if sailing inland.  Ideally you do want a sail cut for your hog-stepped rig but they are not easy to find second hand.