Get out of the car and go to Boots (leaving the other occupant still in the car).
Come back from Boots and get back into the car.
Try to start the engine.
This confused me too, as the immobiliser was the first thing I looked at when the car didn't start - the green light was on and the keypad didn't do anything (because it was already unlocked, wasn't it). Eventually took the key out and put it back in again, and this made the red light come on, which made the keypad accept the security code, which made the green light come on, and the car started first time after that.