In a situation of this sort it is both useful and tactically a good idea to make sure that you can demonstrate that you have gone to law as a last resort.
A good move is to talk to a solicitor and find out what your legal rights are. You say you share ownership of the freehold but each flat will also have a lease - and the lease should say something about the garden. You need to find this out.
Then before a solicitors letter is sent, you send a final one, from you, but with a contents suggested by the solicitor.
I think you may find that legally the person you have the complaint against is the lease holder of the flat and not their tenant.