There are three main associations that landlords belong to. These are The National Landlords Association, The National Federation of Residential Landlords and the Residential Landlords Association. Most of these associations are related to private sector rentals, not the social sector, this is dealt with in the Public property section.
The National Landlord Association

This is an association aimed at private landlords to help promote and to protect the interests of landlords in the UK. This association is really to support landlords only, rather then as a supporter for the public. However, a landlord that does belong to this organisation is expected to adhere to a code of practice which looks after the tenant.
Web: http://www.landlords.org.uk
National Federation of Residential Landlords (Merged with NLA)

This is a federation of local networks of landlords which cover the whole of the UK. Membership is based on an 'affiliate basis', people join the local network on a grade A (individual) or B (company) basis. As with most associations, they have their own code of practice, they also operate an NVQ qualification for a Certificate of Competence for Landlords.
Web: http://www.landlords.org.uk
Residential Landlords Association

This association offers benefits for the private landlord or company. The association offers benefits to its private members such as a magazine, training course and discounts on things they need to run their business. They do however have their own code of practice.
Web: http://www.rla.org.uk/
These bodies are more to help landlords than to help the public, so it is best to look through their websites to see if there is anything that can help you with regards to renting. If however you are a landlord or thinking of being a landlord, then you can contact each of these associations with regards to membership and benefits.Contact them:
For Journalists who wish to contact the associations, they are:
The National Landlords Association Tel: 020 7828 2447
The National Federation of Residential Landlords Tel: 0845 456 0357
The Residential Landlords Association Tel: 0845 666 5000