Thanks for your reply Gary.
Yeah, it's very weird. I've tested it over the past couple of days and can't identify the cause. I only have Orbx FTX Global and England, EZDOK, GSX, AS Airbus and a few airports installed so I'd be surprised if it's overloaded.
I've tried default aircraft, disabling my other add-ons, locking/unlocking the framerate, enabling/disabling VSync and GSync, reinstalling the scenery, trying different animation timers in the options, messing with various settings in P3D, fresh CFG file, reinstalling graphics drivers etc. No joy.
Just seen it at Stansted too.