Thanks for the response Frood!
Checked all of the above, and was fine. The issue seems to be kind of rare but we came across it again just last night. This time it was an animation and oddly some computers rendered the scene with xref's and others without (all computers had servers running to grab separate frames from the same BB job). Checked all have the same version of software etc and other than that, I'm at a loss as to why this might be occurring.
In regards to the UNC paths, just to see I've got this right, the drives are mapped but contain paths such as Z:\1.0 Projects\etc. Considering we've been using Xref's for a few years under the same structure, I wouldn't have thought this to be an issue
Thanks!