Sec 21 mistake

Tenancy Deposits

Hi I have a few questions.
I've made a mistake by not lodging my tenants dep in 2009. However I do have a letter stating that I will keep his brothers (previous tenant) deposit until the council have paid his first months rent then I gave him back his own deposit. all this is in a letter and is legit but not legal. Does this forfeit the sec 21?

I cannot find signed copy of ass tenancy agreement. I know we completed one but I cannot find my copy

I served my tenant with a Served sec 21 Nov 20th 2020 so I have 4 months to go.
I really need to get rid of him as he has been growing cannibis and the neighbours have been complaining. so the police arrested him but he managed to get rid of all his gear and growing paraphernalia before they came to his/my flat. He has been a good tenant but is a real nuisance now with neighbours completing forms from the Environmental dept and logging their complaints with the police