For filling try Toupret TX110 (I think that's what it's called). It's easy to mix, drys quickly and sands well. It does take some time/skill to get previously papered walls looking good though.
Also Red Devil One Time if you need something which is slightly flexible but also sandable. I used to use flexible spreaders for filling, but now prefer a plasterer's trowel. Also, wet all cavities with mist to make sure you get a solid bond. Saves it cracking and falling out later.
For filling try Toupret TX110 (I think that's what it's called). It's easy to mix, drys quickly and sands well. It does take some time/skill to get previously papered walls looking good though.