I have found the culprit, and it was staring me in the face. It was a corrupted terrain.cfg.
I recently posted over at ORBX about a problem with some odd terrain features using FTX Open LC EU around Heraklion such as fields going up steep hills and buildings sitting on the side of hills, actually planted by the wall of the building.
http://www.orbxsystems.com/forum/topic/112191-open-lc-eu-fields-etc-on-hills/
JV at ORBX answered and suggested it looked as if my terrain.cfg was corrupted. Over a couple of days I reinstalled FTX Global base and Open LC trying to get my terrain.cfg back to a default or correct state, but it seems to have made matters worse. Just tonight I tried installing the default FSX Steam terrain.cfg from one of the ORBX backup folders, and that did the trick. I flew about 5 test flights tonight from Jersey to Guernsey and every one crashed at the same point until I installed the new terrain.cfg.
I'm not sure how it became corrupted, but I'm pretty sure it did, and now the problem seems to be fixed; knock on wood
Cheers,