HI Steve. I already had excludes for the whole area. The issue turned out to be that I had the airfield at 33ft to fit in with the Orbx England scenery in P3D4, and of course it's at 55ft in the vanilla sim. Nevertheless, I was surprised that the process of deleting the stock airfield appears not to have worked as well as it might! Also, disappointingly (but perhaps not surprisingly) the Dominies and Jetstreams I have working in P3Dv4 do not work in FSX. I won't lose any sleep over that; I'm sure FSX users will have their own aircraft libraries they can use.
Anyway, it now seems to be working fine in FSX - I will let it run and check that civilian traffic does actually avoid it. The only aspect I'd like still to tackle is the 22/202Sqn Sea King parking and traffic - which is probably easier to achieve in FSX than P3D4. I just need some details of their parking spots.