The Deregulation Act 2015 - Section 21 Notices and Revenge Evictions
The provision in the new act is there to protect tenants against retaliatory evictions of the kind that take place when a tenant complains about the maintenance or condition of the property. Landlords already cannot proceed under Section 8 if there are outstanding repair issues, and this brings section 21 into line with that.
It's not just a matter of a tenant making a complaint - the local authority must been out to inspect the problem and have issued a notice that the repair needs to be carried out for a Section 21 notice to be deemed invalid. If the council's Environmental Health department is busy then there might well be time for a landlord to issue a Section 21 notice and obtain a possession order before they even come out to look at the issue.
In case you are reading this and thinking 'well I'll just issue a Section 21 at the beginning of the tenancy then', you now can't do that either! You cannot serve a Section 21 until 4 months of the agreement have elapsed.
Your Section 21 will also be invalid if you hadn't shown (and can prove you had shown) the tenants the EPC and Gas Safety Certificate at the time they viewed the property.