I was thinking about it but all the other airport sceneries are also Inbuilds. (EGLL, EGSS).
I also have the free EGXU (Duxford) but this is way north of EGGW.
I have Orbx's region GB central and north mesh but again, I had no issues with any add on scenery at that region before.
I assume I can start removing add ons until I get to the one which is causing the problem (or not) but it shouldn't be that way. So, I'm still puzzled.