It will only belong to the Council if they did not include it in the area they sold off when they sold the first floor flat. The Council need to give you a plan showing the actual boundary - or you could get one online from the land registry site. The land clearly does not belong to you as you are renting. If the Council has sold it with the first floor flat there is probably nothing you can do, but you could contact your local Councillor & ask them to take it up with the officials.