When to complain
If a letting agent doesn't resolve your complaint within 8 weeks, you'll be able to complain to their redress scheme. If your letting agent gives a final response to the dispute within 8 weeks, you can complain to their redress scheme after you receive their decision.
You may prefer to complain after you have left.
You must complain within the time limit set by the redress scheme. Letting agents must cooperate with any investigation and agree to pay compensation promptly if the redress scheme awards it.
The letting agency has to comply with any orders to pay you compensation, to pay a fine or to put something right. Tell the scheme if the letting agent doesn't comply.
Source: Shelter 2015