Hi Mark.Many thanks for your reply. To clarify.

I do not use the MAN switch on the default CIVA INS. I leave it on AUTO.

The routing I had, FP created by PFPX, EGLL-LEMD had me departing from Rwy27L at EGLL, but I did not put the HDG to 271. Instead, I set it to 137deg as that was the first turn after lift-off, and it did follow that track after setting the VOR for LON at 113.30.

Then, as per your tips, immediately after lift-off, once I had climbed to 1000ft, I engaged AP after already having set the FD to SYNC in the before taxi checks, set the HDG switch on, as well as the NAV switch, then moved the Navigation Panel Mode switch to FMS.

The aircraft followed that 137deg heading but then just carried on going Eastwards towards the North Sea when it should have gone South towards Madrid.

I then switched from FMS to VOR and brought her back to follow the FP South-bound track, but when I switched back to FMS, the aircraft immediately veered and reverted to a 177 deg heading. Nothing in the clicking that middle right-screw next to the display showed up as a 177 course heading.

So, now I am quite bereft of ideas which way to go.

Wish I could attach images here.