As regards the garden, it depends on the documentation in place.
There is a lease of each of the top flat, I assume. Possibly also the downstairs flat though this could be materially irrelevant if the freeholder is owner occupier.
Is it the freeholder or upstairs flat owner who has use of the rear piece of the garden you asked about?
Do you know who actually owns that piece of garden (as in the name on the title deeds/Land Registry)?
If the upstairs flat occupier, do they definitely own (rather than renting) and, if so, does their flat lease (if there is one - should be if they own it) include ownership of the relevant part of the garden or simply a right to use (or is there a separate lease of the garden land and, if so, who owns that?).
If there is a lease, it would also make a difference whether there are any provisions affecting the relevant piece of garden as to who is liable for any problems.
Checking the title registers and lease(s) should shed more light.