Sorry Mark, I tried you suggestion, followed it to the letter but the aircraft instead of following the PFPX-generated Flightplan, which I imported into the sim's Flight Planner and checked the route was being displayed, instead of following the 269 deg course, it rigidly stuck to course 177 deg.

When I tried to revert to VOR and chose HDG to 269 the aircraft changed course to that heading, but the minute I changed VOR to FMS, it went back to 177 degrees which is nothing like the course.

At that stage, I packed it in. I'm now at a loss to get this INS to follow a flightplan.