Thanks. Seems to me that FS2020 just assumes metres regardless of what the unit is set to in Blender. My model is 10% scale in Blender with units set to MM and I’m now importing at the correct size when scaling to 0.01 in the scenery editor. I might well resize in Blender to make it more intuitive, but for now it works.
I have changed the units in Blender to pretty much everything, but it makes no difference to the size that FS2020 imports at.