I ended up going through a full rollback process with the help of this article…
After initially re-configuring my default to 5.8.0-53 I eventually binned 5.8.0-55 entirely. I’ll run the upgrade again once this is all resolved and the newer drivers are being shipped with that upgrade else I’ll be stuffed again and won’t be able to get anywhere to fix anything.
This is running with the 460.73 NVidia drivers - all is well
I’d recommend for anyone on Ubuntu 20.04 to get ahold of 5.8.0-53 if you don’t have it already and try that out - If it’s all good, bin off the newer kernel until Canonical and / or NVidia have resolved this mess (which could take a while).