I've now got the car back and all is working as it should.
New engine oil in (10w-60 PETRONAS, it burned through the 0W-40 Mobil 1 in had in before), new gear oil (MTF with KAAZ additive), a new speed sensor for the gearbox and no fault found with the fuel sender. I've only driven it home so it's on half a tank now, I'll have to see how the gauge behaves. Tom said that by taking apart all the contacts it may cure it, we'll have to see.
Once I got it home I put the track wheels back on as I've sold the bronze Fighters. The bloody jack fell off the subframe, slight moment of panic as it was close to the sump. Turns out it's hit the bottom of the bracket that holds the front bar on that has the bonnet catch on so the bonnet wasn't shutting properly. Cue 14 stone later by me standing on it and it seems to have mostly fixed it.