Since I wrote originally I’ve installed the latest SDK, and tried with an airport not just a single object. I used a polygon to exclude everything under it, following the steps by BenneyBoy444, I had no errors and after a reset it still doesn’t show.
And as you asked earlier, yes I have been doing a reboot after each attempt, which is a pain because it takes quite a while to run up the sim each time. In answer to you other point, the first attempt was at an existing airfield, just a small one which I though would benefit from a couple of buildings etc.
I’m getting close to thinking there’s either something wrong with my installation of MSFS2020, or I’m missing something so obvious it’s driving me nuts… 
Thanks again for your help! 