After a bit of head scratching I’m up and running!!
The few errors I had were due to (or at least they went away after I deleted) other VirtualBox Machines, and an invalid exports file (which it turns out Local by Flywheel was still listed in, so I deleted those lines and left the Vagrant bits).
One thing I hadn’t realised was that Trellis would set up my Bedrock .env file automatically for development. ‘Proper job’ as we say down here in the deep south of England.
Next mission is to deploy to Kinsta, and then convert my current vanilla Sage 8 site to T/B/S9