I've tried Google and Reddit and no luck.
I was around 13 years old when I got it from the library which would have been 1993/1994
It's a group of children that spend some time wandering round London, stealing food etc but they all live in the underground.
They go down to the underground and get to their living quarters by going through the tunnel once a train has gone and then the network where they live is accessed through the doors/open arches in the underground tunnel walls. They can move across the whole of London in this network.
I absolutely loved the book but cannot remember the title or author and would love a copy to see if it's as good as I remember!