My neighbour recently undertook some works and at about the same time I noticed a leak to my conservatory roof. I cannot find where the water is coming in from but it’s too much of a coincidence for it not to be related to the works. Is it my neighbour’s responsibility to investigate? He doesn’t seem to think so.
If you believe the leak to be related to party wall works (I should stress that is the work covered by the Party Wall Act rather than the general works) and your neighbour is denying responsibility you have the option to refer the matter to the appointed surveyors for determination. The risk with this course of action is that the surveyors determine that there is no link and that you not only end up paying to make good the damage caused by the leak but also the surveyors’ fees.
Referring a dispute to surveyors can be time-consuming so the priority should be to resolve the leak thereby removing the risk of further damage and mitigating your losses. With that in mind, I would suggest organising the repair yourself but recommend that you ask the contractor to write a brief report providing their opinion on the cause (and take photos if inaccessible). This needn’t be anything comprehensive, just a couple of lines written on the invoice.
The contractor’s opinion will help you decide whether the defect is actually related to the party wall works and, if so, will be an important piece of evidence that should be passed to the surveyors (it may even encourage the building owner to accept responsibility and avoid the need for surveyors’ involvement at all).