UV Bake, Black line around margin?

Iv been working on this project for months and have done literally hundreds of bakes with no issues, then yesterday, for no apparent reason the UV Baker started adding a 2…3 pixel wide black line around the texture space islands, and i cannot for the life of me work out why!

before
http://www.tv-consultants.com/OOPz/temp/UVBakeBlackEdge0.jpg
after
http://www.tv-consultants.com/OOPz/temp/UVBakeBlackEdge1.jpg

im trying to re-render sections of the texture (without clearing the whole thing). if i set zero margin, it renders as expected (only the result is unusable due to the messy edges that leaves).
if i set a margin > 0, it adds the margin as it should, then adds another 2…3 pixels around the outside of that like a second margin, only black.

this of course makes a horrible mess, intruding into close sections and making the mipmap versions look horrible

… any ideas?

(WIPhttp://www.tv-consultants.com/OOPz/l115a3.php )