There are a lot of opinions here that go from wrong to outright judgemental. I have explained it once and so has andy l. So I will put it simply.
The leak is from one flat to another.
Leaks and the damage they cause are covered by building insurance and not contents insurance .
There will be one building insurance in place to cover the whole building. This will be administered by the freeholder or the agent acting on behalf of the freeholder. So You and your neighbour need contact the freeholder or agent and inform them that you want to claim and the building insurance should take care of both fllats. The <span style=”font-size: 0.8rem;”>he agent may try to fob you off but this is the law and citizens advice will more than happy to advised you if that is the case. </span>