I had a look on WIS and the sensor is definately on the differential, not on the rear of the transmission as I thought.

On WIS the connector shown isn't a plug and socket as you would think, its a terminal block with ring connectors.

Also, it says that you need to remove the seat bench and backrest, so if you haven't already looked behind the backrest, it could be there.

Looking in the fault table, the fault for 'ABS lamp coming on whilst driving and does not go out again' is 'Wheel speed sensor cable interrupt'

So again, you really need to check the sensors and the wiring. Sensors need to be checked with an oscilloscope for a definative answer, but as previously suggested, a multimeter set onto an AC scale may give a reading when the wheel(s) are spun.