My advice would be to seek the requested deed from Camden. They have a history of changing policy and withdrawing what has previously been agreed in writing outside the lease. I also suggest you check what the lease says about service charges in respect of the garded and any walls/fences around it. Are you liable for their maintenance? What is the evidence Camden ever suggested the occupier of the upper floor had any rights to use the garden, whether you or the tenant?
I suggest you see your Ward Councillor to seek support and (if possible) raise the issue as a Member’s Enquiry which gives it some precedence.