Is it criminal damage if you punch your punch bag in your flat......

[PeasantCrusha]

.......and it breaks the neighbors wall?

The neighbors wall being crumbly and weak, and getting damaged via the vibration.

My wall is fine so I dont know how it could damage her wall, which is after my wall.

I had mine hung up in a door wide doorway. Used to butcher it daily when I lived in uptown. Never heard any complaints.