I've now studied the Special Constable poster in more detail (see blogs passim) and can reveal the idea is even more complex and bizarre than I first thought.
You'll recall that a Special is on some seedy looking street corner, just yards away from where a drug deal is in progress. He's keenly observing events and looks ready to take some kind of action.
The copy says that Specials wear uniform and always have back-up. But this guy is on his own in plain clothes.
Perhaps he's off duty but just can't leave the policing thing alone? Once a part-time copper, always a part-time copper.
But wait. There is back up. It comes in the form of the constable's own reflection in a puddle.
If he took his eyes off the criminal activity for just a second and took the trouble to look down at his feet, he'd see that in Puddle Land, he is wearing a uniform and carrying a radio.
There's enough material here to fill a semiotics seminar for a day. If I discover any further layers to the creative concept, I'll let you know.