I don't know who or what is doing this. I think there is too much of it to be ants but when I think how old that flint wall is it makes me very angry to see it like this.
A flint wall is usually built up as two skins. The centre of the wall being filled with rubble and it is possible that the weather, ie repeated frost and rain during the severe winter could have caused the breakdown of the wall. What do I know! I just happened to show the photo to my hubby, a retired builder and got an ear bashing on the life of a flint wall!!