Landlord taking away use of garden

Tenant26

The rent is in the same region as other similar properties in the area. 

saajan_12

Sounds like this is a communal garden between all the flats. So you wouldn't have exclusive use of that space, at most you'd have shared access to it. As such, you don't have the same expectation of enduring utility as you would with spaces that are exclusively yours - what if another tenant had a bbq out there? At worst, it was nothing to do with the lease and you had no permitted access, so anything to date was just not enforced. 

Practically it'll be hard to enforce rights to something you didn't necessarily have.. I'd just decide if the flat with no outside space is still okay wiht you, or look to give notice and move. 

Tiglet2

I'd second everything @saajan_12 said.  If this is a real problem then you might be better off serving notice and moving to another home with a garden.