Fixed.
I've detailed the process below but can anybody tell me where I can buy a single VW rear number-plate LED light from? All the ones in ebay are copies or come in a pair (at £50). Any help appreciated.
For anyone who has the same issue, this is what I did: (I'm posting this as if I knew what to do, I could have it done in 20 minutes, as it was with all the trial and error it took me over an hour!!)
Lying under the car at the rear with the lights on, I could see the lit LED light inside the bumper. It was slightly cracked so I guess that someone knocked the car when parked and it popped it through the bumper.
Reaching from below the car, I was able to get fingers on the light but couldn't angle it to push back through the hole for the light. After about 20 minutes, I gave up doing that.
I undid the 3 small torx bolts which each held a single plastic clip in place on underside of bumper. Popped the 3 clips out and with that and also undoing one medium torx bolt I was able to flex the bottom of the rear bumper back enough to get both arms in.
After a lot of messing about, I worked out that the bets thing was to disconnect the power cable. To do this, I had to remove a red tab which kept the two small retaining arms in place. With the red tab gone, I could lift two small retaining arms and slide the power cable off.
The light fell down through the bottom of the car and I was then able to push the power cable through the hole for the light and connect to the bulb which was now on the outside of the bumper again.
Once connected, it was just a case of popping the light back into the hole.
Job done.