We managed to get some of the free tickets to the Sky Garden at the top of the Walki Talkie. It is an indoor roof garden with lovely spaces and a green wall outside. You can book a free visit here but if you book a table for coffee/breakfast etc. at the Sky Pod bar you are right in the middle of it. Coffee was good and very reasonably priced. There are also other restaurants on the same level. There is a lift up to the coffee area so you can see much of it without walking and stair climbing. In the summer there is an open area. 
De Beauvoir Gardeners 