Cracking in the unified renderer

for my WIP at home i am rendering the opening starwars crawl (but themed for an event), I have just modelled the spacecraft and rendered it with both the stock renderer and the unified renderer however there is a problem:

in the default renderer when rendering the winshield it comes out perfectly (within reason) however for the unified renderer there is a slight gap between two of the triangular polygons that makes up the rectangular polygon - that makes up the windsheild.

this crack is about 1-2 pixels wide.

does anybody know how i can get rid of this???