Ok, no need to post another blurry pic, found - accidentally I might add - that the connector plugs in to the rear of the gearbox ... so I'm assuming that it's perhaps the reverse light switch. Anyway, mystery solved.
The other mystery, as to why the car starts and then immediately dies, is somewhat mysteriouser. Today I went to a scrapper and acquired a crank sensor from the same engine type, put it on ... and it fired straight away and kept running. Problem solved, I thought. But no: I turned off the engine and tried to restart, and same problem again. Starts and then dies immediately.
So I went and downloaded a 'free', trans. 'very limited functionality' OBDII program, to use with my ELM327, which reported P0340 Camshaft Position Sensor. A fault somewhere on the circuit, possibly with the sensor itself?
Before I start thinking about possible nightmarish chain-stretch issues, I suppose I should change the sensor. Does anyone know where this is, or they are, if there's more than one?