Thanks for that Frank,
if I'd looked carefully at the unit, or at the photo I attached, I would have seen the pictorial representation of an axial leaded component moulded into the body.
A bit of investigation revealed that it is not just a resistor though, it is an NTC Thermistor. NTC=Negative Temperature Coefficient - i.e. resistance reduces with increase of air temperature.
To test if that actually was the fault causing the system drop-out I fitted a 2K resistor between the tags. Lo and behold, as long as I'm on the move with air not much above 25 deg C (it's currently in the mid teens over here) going into the air box everything is rock-solid. When I stop, and keep the engine running the system eventually drops out. I guess that's the under bonnet temperature heating up the air in the air box and the resistor not dropping in value as it should.
Not to worry, the exercise has proved the point and, as I can't be sure of the exact specification of the thermistor, I guess the answer is a new sensor, as you said.
Once again, thanks for pointing me in the right direction. Much appreciated.