Speed Sensor
 

Do I need this? It had a wire going to the ecu which I've removed which makes me think I can remove the whole thing? It looks like originally it was just for the ecu & fuel computer?

No Need for it.

Nice one, am I right in thinking its the 3 pin multiplug that goes on the gearbox? (not the big circle black one)

It is the three pin plug. The other is the reverse light switch.

You have to leave the sensor in the gearbox otherwise the oil will come out.