uncooperative neighbour

We have a problem with damp in an interior wall. This is an ajoining wall to an exterior wall of our neighbour. It is probable but , not certain, that the source of the problem is moisture/water trapped below our neighbour's decking.
I spoke to neighbour/owner who no longer lives in the property. about the issue two months ago. Since then I have tried to improve the look of our wall.
I now notice that the problem has returned and have no evidence of action by our neighbour.
Do I contact the Commune, send a recorded delivery letter to our neighbour, contact our Insurance company or all three?
All advice would be welcomed.
Thanks very much
Cheesed-off in Brussels