[wraymond]

You can be chasing sound penetration for years. It comes down to the frequency and resonance of particular sounds/materials and elimination is difficult.

I sympathise, it becomes increasingly intrusive as time goes on. I experienced the problem some years ago, in the end getting a firm to take readings and suggesting effective materials/methods proved a quicker and more successful way to go. There just might be records of the construction of the houses held at local authority offices which could show party wall build.