Just a quick update out of courtesy as you took the time to try and help.
I bit the bullet and factory reset my entire (a rather grand word for one router, one switch and one AP 
) network, my Proxmox server and hence both my UniFi controller VM and my HA VM.
I have to say that 24hrs in, everything looks rock solid.
I’m pretty sure that the UniFi network was messed up with all the migrating and IP changes and reuse.
Anyway, thanks for your input, sometimes it is really useful to have a different view on things even if it doesn’t provide a direct solution. In this case it at least gave me a bit of a sense check that I probably wasn’t overlooking something obvious.