Frustrated

Sorry maybe I should have highlighted that we have already done pressure and meter tests which confirmed that the potable water pipes are not leaking in both houses, hence it has to be failure of the waterproofing layer on my neighbour's side. Due to the almost regular appearance of water (in our master bathroom walls) at around 10am plus everyday, we have also eradicated the possibility of external water seepage eg from rainwater. Hence, the contractor has recommended hacking and re doing the waterproof layer. How can we enforce this, instead of allowing the neighbour to take the cheaper but not lasting solution of injecting his toilet wall with chemicals?

My next door neighbour (terrace house) master bathroom is leaking water to my master bathroom (our bathrooms are back to back) and we have confirmed that it is very likely due to failure of the waterproofing layer in his wall tiles in his master bathroom. Water keeps appearing above our mixer in our master bathroom, despite the fact that we have vacated the house since more than 2 months ago, hence no water usage at all Contractor has advised him that best and longest lasting solution is to hack his tiles and re do the waterproofing layer, but neighbour refuses to spend the money and wants to opt for the cheapest solution - flooding his bathroom tiles with chemicals, which does not come with any warranty and following which the leak is likely to come back very quickly. Is there any way we can enforce that my neighbour takes the permanent solution of hacking as we do not want to keep re-visiting this issue every few months? There has been massive delays and we are contemplating legal action. Any kind souls out there with experience in similar situations, really appreciate if you could share your opinions?