ExSp33db1rd it can only show a heading relative to its own fore and aft axis, can it not ? If it isn't bolted along the fore and aft axis of the aircraft it won't show the aircraft heading
Nonsense. It doesn't have to be bolted to the aircraft at all. It has a compass needle, so if one aligns the lubber line with the fore aft axis of the aircraft (fairly easy to do with the mechanical bits already attached to the aircraft in that axis, then the needle does indeed point at magnetic north and the aircraft heading is shown under the lubber line.
Of course, it could have compass needle, which would point to magnetic North, but that's not the same.
As heading is relative to magnetic north in this example, then it is the same, all you need is a calibrated rose to show the relative bearing from magnetic north and the lubber line and there's your (magnetic) heading!
.. the track made good
Rubbish, TMG is the resultant track over the ground when a heading is combined with wind aloft and is effectively your track over the ground. The GPS map will show that whilst the lubber line (if the device is properly aligned) will show your heading.
Educate me.
I hope I have, else Google is your friend.