Problem diverted!
Im not too sure if it is a bug or something Ive done?
Basically I had an assembly (1 Housing + 2 Components + 2 Adhesives) with 2 simplified reps. The housing was common to both reps.
I had a succesful run with rep 1, but when trying a run on rep 2 this was where I got the problem?
In the end I got around it by just making a copy of the assembly and having a single rep in each.