A Property Redress Scheme is an independent scheme that administers a complaints process for tenants that have a problem with an agent (whether it's a let-only agent or a managing agent). There are three government-approved Property Redress Schemes, and all letting agents (i.e. enyone who lets or manages a property for a separate landlord) in England & Wales are legally required to be registered with one.
A Property Redress Scheme considers complaints against agents registered with them on grounds things like poor service, incompetency, failure to follow proper procedures / obligations, unprofessional (rude or discourteous) service, unfair treatment, avoidable delays and in some cases deposit-related issues. So if you're having a problem with a letting / managing agent along the above lines and you've not been successful in resolving the issue yourself, you can lodge a complaint through the Property Redress Scheme that they're registered with. The Scheme then has the power to take various courses of action, such as require the agent to correct their mistake, order tehm to issue an apology or in some cases pay you financial compensation.
First, check with scheme the agent is registered with. Usually it's mentioned on the agent's website, but if not there is a members list on each of the three schemes' websites that you can check. Once you've found which scheme the agent belongs to, you can lodge a complaint by filling in a form online via the applicable scheme's website.