Write to your landlord saying what is wrong and giving him seven days to fix the problem.
If that doesn't work, write a second letter saying that, as he has not done anything about it, you will be employing someone to fix the problem and will be sending him the bill and will, if he does not pay up, take the matter to the Small Claims Court.
Get the work done and pay the bill. Make sure you get a receipt.
Send him a copy of the receipted bill with a covering letter giving seven days to pay or you will take him to the Small Claims Court
Write saying that you are taking him to the Small Claims Court.
Send all letters recorded delivery and keep a copy of them.
The Small Claims Court is not expensive as neither side is allowed to have solicitors. The fee is about £100 which is added to your claim, so the other side pays all your costs if you win.
If the other side does not respond when he receives the claim documents from the court, you automatically win.
If yo do have to go to the Small Claims Court, make sure you have copies of all the documents. You should also have information as to when the problem started and the steps taken to cure it, e.g dates when the maintenance men visited, phone calls made etc. A copy of your Tenancy Agreement would also be helpful as it will set down who is responsible for what. (I know it's the Landord for your problem, but the Court will want to see that in writing.)