Hi Michael,
Thanks for your reply - I have been away so no had chance to try anything further yet I am afraid.
I followed the TLS video (MeshCommander - TLS & Mutual-TLS - YouTube) for mesh commander, and the certificate is issued using the software for TLS, so I assumed it would work for MTLS also. I did not realise anything further was required (the video i used to successfully set up TLS doesn't suggest anything further) - I'll have a look at the document you link to when I get chance and see if I can find a solution.
Ultimately, I would like this to work on the Mac - I have it working on a Windows machine at the moment, but the Mac is what I use day to day.
Best regards,
Jon.