There's no point in you writing them a letter.
Contact Housing Services at your local Concil. You need to speak to the Housing Renewal Officer (or equivalent).
You would like the Officer to inspect your property because your landlord is not maintaining the property as a safe and healthy environment for any potential occupier or visitor. You need to use those words. They'll then ask you to outline the issues.
If this is genuinely a leak (and not just you p*ssing on your landing

) Your landlord is in breach of the Housing Act 2004: Part 1 - Housing Health & Safety Rating System. The officer will ask you to confirm who the landlord is, then write to him to offer him the chance to be present for an inspection, then inspect the property with both parties present (if your landlord turns out). Once that's done he will issue a schedule of deficiencies to your landlord. The landlord must resolve these deficiencies, or he will be taken to court and have a statutory notice issued on him to force his hand and he will be charged for the privilege.
Probably the first visit and notice will be enough to get him to sort it out, or terminate your tenancy.
Even if he terminates your tenancy he will still have to remedy the deficiencies.
This is the correct way to get your defects remedied and no, your property doesn't have to be anything to do with the local authority to do this.
My recently departed tenants pulled this on me because they wanted the back door replaced. The back door has been replaced, but because of the problems they were causing by their keeping all the windows shut and storing a huge amount of shyte in the house, that were highlighted during the inspection, they had notice served on them.
Tenants: just as c*nty as landlords.