Repairs in a new built flat

RaNDoMuSeRLoL999

Hello, 
I purchased a new flat in the end of 2018, the flat was completed end of 2017 or beginning of 2018. This is a lease so I am responsible for the indoors property and the landlord is responsible for the outdoors.
Within a month of moving in, I noticed it was some watermarks in the ceiling when it was raining. I have been reporting to the landlord every time I have seen those watermarks.
What started being watermarks has become a leak. The leak is damaging the ceiling which is plasterboard.
The landlord has come a number of times with professionals but failed to repair it, the leak is getting worse each time that rains. It affects to our sleep as part of it goes to the bed.
What I can do to fix the issue as fast as possible?
Should I look for professionals to fix it and pass the bill to the landlord? Should I deal directly with the insurance of the building?
Should I court anybody?
I don't feel the issue is taken seriously I am desperate to fix it whatever way is required.
I bought a new property because I wanted to live in peace, I wanted to live with no problems at home, this issue is stressing me and affecting my personal life and at work.
Has anybody been in a situation like this? what is the best I can do?