Hi,
I live in Ramsgate, Kent and have exactly the same experience. It is singing away when I walk the dog last thing at night - around midnight. Have heard it for about a week, always in the same place - appears to be coming from under a neighbours car, on a concrete drive !!
At first I thougt it was some kind of alarm as it can be heard quite a distance away, but when you go near the sound changes and sometimes stops when the security light goes on. If they have seen e peering under the car the neighbours must think I'm trying to steal it or something !!!
Sue