I love a happy ending. The leak has gone. I tracked it down to the pipe in the wall near the inside stop tap. Dave from next door came round and pulled out the offending item.
Now that is what I call a hole. The water was really spurting out once we got the plaster off it. Turns out that sometimes the flux used to solder the pipe connections tends to corrode the pipe itself. Which is not good.
This is what it looks like now. Much better. Thanks Dave.
The only thing that worries me is that I have a house full of this pipe, so I suppose I’m going get this every now and them. Ah well, at least I have one less now.