I finally got around to changing the knock sensor today. It's a fairly quick job, the only tricky bit is getting the torque wrench in there.
I have been driving up steep hills and accelerating from 1500rpm to see if it still pinks - there are a couple of pinks then it stops. This is much better than before & I'm satisfied the sensor is doing it's job and stopping the pinking when it starts.
Incidentally, if anyone is thinking of changing the sensor, it's worthwhile taking the old one off before you buy another and see if it's dirty. Mine was well filthy and I reckon this is why the sensor wasn't working.