If you're subjected to harassment, the matter should be reported to the Tenancy Relations Officer of your local authority.
Or, if you feel the harassment to be severe, you should report it to the police.
Ask if the landlord is a member of a professional body.
Landlords who are members of these trade bodies will have a Code of Practice that they must adhere to, so you should be confident that your rental will be professionally managed.
If your landlord is a member of one of these professional and you're having problems with the landlord, you can report him to one of these bodies.
Check the Code of Conduct, and when writing your complaint outline where you feel your landlord is in breach of the code
Also local councils, colleges and Landlords Associations run accreditation schemes that set minimum requirements for your landlord to comply with.
Details on these can be found from the Accreditation Network UK (ANUK). Ask your landlord to show you their accreditation scheme membership card or certificate.
If you think your landlord isn't complying with scheme, you can refer your complaint to the scheme operator for arbitration.
You can always take your landlords to court if you've had to pay out for a repair which should have been the responsibility of the landlord.