Serving Section 21 before its banned

Possession - Section 8 and Section 21

I want to give S21 notice to a tenanted property which is managed by an agent who is pretty useless. The tenancy began in 2017 and the tenant has refused to pay even a penny more since that date.  The EPC was valid in 2017 because it was done in October 2009.  I read in an engineers note that its valid for 10 years OR until a new EPC is done if its the same tenant in the same property.  My question is is that right or should I have got a new EPC in 2019?.  Does this mean I cannot issue a S21? Can I do a EPC now or rather shall I get a new EPC done now or is it too late?.  Does the fact that it was valid when I first rented make it OK?.  Also the agent has got 2 months deposit which he has deposited in 'My depoits'.  As he had done it before the new rules for 5 weeks rent only came into force, is it ok if he returns part of the deposit leaving only 5 weeks in 'My deposit' which is what the deposit company has asked the agent to do or should the full deposit be returned before I can serve S21?.  Please clarify.