Deed of surrender of tenancy

Possession - Section 8 and Section 21

I’ve been trying to possess my property for two years. The property is managed by Martin Co letting agent. The tenant himself is lawyer himself. I had tried the legal route with no avail. Long story short he agreed to move out on 15th Oct. Two weeks ago he emailed to say that he’s moved out but he wouldn’t hand over the keys unless I sign a deed of surrender of tenancy which he is drafting. He still hasn’t emailed. I guess he just doesn’t want to pay the rent for the period he had stayed after 15th Oct as he has stopped paying from October. His emails are so vicious. The emotional abuse has been the worst. I don’t dare to ask as he would come back with nasty words. I don’t really mind the rent. I just want him to go but he seems to be so difficult. A friend suggest I change the lock.
What shall I do?