originally I tried the BREW service, and then I tried directly using mongod.

Interestingly there were no issues before I added authentication to mongodb - I had added it on the P
C because ultimately in deployment I would need it but the authentication would not work and (from what I read) this is an ipv6 issue on Macs.

I uninstalled, and reinstalled (via BREW) and it worked once through the brew service but not after that. Honestly the decision to give the PC a static IP address was not only simple but also clearly the correct solution. I was being daft having two different DBs but I am new to this and it seemed obvious to have it installed locally until I thought about it and realised it was a poor solution and a shared networked instance is logically much better.