Update:
He said he was followed by two marked Police cars today, on seperate occasions for some miles, but neither stopped him... he must be well and truly on the system now. Said he's been on the look out for Police, but didn't see either of them approach him... must have been tucked up somewhere and pounced.
I just asked him about the caution thing, and he says, the yellow/pink stub which he had to sign says: CAUTION: Then written in: EXCESS SPEED, name, reg, time/date etc. And something along the lines of, usually you would be prosecuted for this....