Solved this just before posting, but thought it might be useful if anyone else comes across this problem.
The problem is where the model is showing diagonal lines all over when zoomed in, and also the edges of objects are jagged, pictures are attached.
The solution was that there were objects very far away from the main part of the model, deleting them fixed the graphical issue. I used "Fit to objects" to find them.
Bit of an odd thing is happening on one of my models, I'm not sure if it's corrupted in some way or theres a setting which I've accidentally changed, but it seems to have a hatch over the whole thing when you zoom in past a certain point.
Attached a couple of screenshots zooming in to show what's happening.
The render setting is openGL, quality set to high. I've tried changing anti-alising but doesnt seem to have any impact
It would be really nice to find a fix for this rather than starting the model again...