Thanks for this Mike, and your work around solution does work for the viewport, its obviously draw edges option on the Open GL options that is the problem.
However... on one file i am working on, we are able to view the model in open gl with edges and no diagonal lines, where as in the file i attached we cannot view the model in the design layer in open gl with edges, so there must be some kind of bug somewhere along the lines that makes these 2 files work differently...?
Lucy.