This is the circuit I've tested (with the additional current limiting diode on the 12v rail that I've yet to add). I'm sure there are still errors or problems with it as it stands... but at least I get a reading! I'll test this more later in the week once I've added the diode - and all being well hopefully move onto stage 2, putting it all together; merging the tsdz2-wireless-controller code with the ebike-app code from the motor controller - add a sprinkling of the vesc bldc library and hopefully we're go!
Edit - the current limiting diode didn't work.. so I've removed it from the schematic below. But I also worked out what the problem was. I had an electrolytic cap between the 12v and gnd to smooth out power delivery when i thought that might be an issue and had left it on the board, but I hadn't noticed that the cap was now the other side of the current shunt... So the glowing shunt was as a result of the inrush of current to charge the capacitor, nothing to do with the circuit itself. So looks like I have a good circuit now to code against