Soft walls - Siporex

I was asked to fit a kitchen roll holder to a kitchen wall in a flat in Broomhouse. The client had already tried to mount it using the three screws and wall-plugs provided. She said that the walls were very soft. Using the provided 35mm screws and plastic plugs was unsuccessful, the plugs being pulled easily from the wall which drilled quite like deep plaster. I had to use 75mm screws to get sufficient grip.
I spoke to a retired architect friend who explained that these walls were probably Siporex which is a lightweight concrete block which is still used quite widely.
If you find that a solid wall drills like deep plaster then it may be Siporex or an equivalent and you will probably need very long screws and some patience.
Harry