However, we do not give access to the assignments on the rare
occasions that requests come through. Assessing the minor source
matches should give a good indication of the source that 'your'
student is more likely to have used, rather than them knowing a
student in another institution and copying their work .

But what about the case(s) of:
2 students from different institutions using the same essay mill etc., & being given the same essay from it.

Students contacting friends at other universities who've had to do similar essays.

I've had cases in the past fo work that was substantially the similar (great big chunks) to that submitted a few years before to a different uni. Far too much overlap to be poor scholarship. I have requested those, and in cases where others have asked, have allowed access.