It is especially critical to deal with complaints quickly. If a tenant makes a complaint about something the landlord must respond in writing within 14 days.
If they don’t respond and the tenant makes the complaint to the Council and the Council issue a notice on the landlord, the landlord will not be able use a section 21 notice for the following 6 months.
It might be that the landlords response states that there is actually no problem, for example it might be that the tenant is complaining about mould in the bathroom, but the landlord keeps finding they have turned the bathroom fan off. But if there actually is a problem it doesn’t have to be resolved within the 14 days if not possible/practical. It might be that the landlords response states that the problem will require longer term work, and/or quotes.
The important thing is to respond in writing within the 14 day period.