Hi James, I happy to hear you at least made some progress. There is no obvious reason (that I can think of) why is should not work for you as you originally specified your setup, therefore I suspect a configuration error, rather than a bug.
If you need further help with with your setup running on more than one box, please take a quick look at
https://osmocom.org/projects/cellular-infrastructure/wiki/ReportingBugs
I don't think in this case it's necessary to supply absolutely everything listed there, but at least config files, and some traffic capture. otherwise you leave others guessing too much and it's hard (impossible) to help.