How far on the fuel light

[limax2]

Ive done maybe 14 - motorway 55mph bricking it all the way home, only to look inside and see what looks like loads left

There is fuel in the tank that can't be used. The reason is that the tap has a stand pipe on it 40mm long, the top 26mm part being mesh, acting as a rough filter, but the bottom 14mm is just a tube. From a measurement I once did with the bike level but stationery it took 1.5 litres before any fuel flowed out of the tap. Of course on the move that last bit of fuel would be swishing about so some of it would get through. Also 0.2 litres of that fuel is below the tap fitting face. If you ever decide to completely drain the tank, to get a bit of water out for instance, you have to do a lot of shaking to get the last bit out. 
Not an answer to the original question I'm affraid  :\