I was involved with them many many years ago when I started doing cat rescue work in London. In fact, they taught me how to trap. Many a happy night was spent in the rough estates in SE London and the East End being pelted with beer cans, stones and bags of chips while we tried to help cats and kittens struggling to eat the food we had brought.
Sadly, this was over 20 years ago and some of my friends involved then have either died or moved away. I've lost touch with how they are doing as a charity - I think there are branches up and down the country. I have heard that Elke de Vries, one of the prime motivators, is still doing work in London but I don't know how active otherwise the Central London branch is any more. Elke is around the same age as Celia Hammond I think so late 60s...
Perhaps someone from one of the Branches will post and let us know.