Thanks for responding Michal, much appreciated.
If you see my opening post you will note that I have run default aircraft with the community folder empty and the problem persists. In addition to emptying the community folder completely I then added MODS one at a time to see if any particular MOD might be at the heart of it. To be clear, there are not a lot of MODS in the folder when they are all there, two aircraft, the Working Title Garmin MOD and about 10 small airfields in the UK and New Zealand.
So, whatever configuration - default aircraft (Cessna 172 & 152) and Community Folder ‘full’ or empty flying MOD aircraft - it appears to make no difference. Flights are around 25 to 30 minutes long, all out of EGCK and termination at EGOV. Avionics on or off, ATC involved or not, AI aircraft on or off, flying at 70kts or 120kts the brief black screen events occur randomly.