OK so I've been working away moving scenery around etc.   I've no idea what's changed but I've just managed three successful loads in a row.   The last steps I did were remove three large OSM files from the custom scenery + the scenery_packs.ini file.   This resulted in it starting.   I then replaced, one by one, each osm file, restarting each time.   Finally I replaced the original scenery_packs.ini   It seems to work for now...   Andy