Leaseholder sharing garden thinking about buying out the other two leaseholders

[Clairesisternotparent]

Well, here I am on mumsnet again. You seem the best place to ask about anything. At all.
I own a flat in a three-flat victorian end terrace house with a long lease (999 years). We are all leaseholders, and pay £11 per year ground rent to an obscure freeholder.
There are two garden areas, one front, one back. All officially shared but unofficially the downstairs flat uses the back garden exclusively (not deliberately fenced, but it's outside their front door.) The front garden we unofficially share with the upstairs flat.
Originally, when we moved in 20 years ago, gardening was paid for out of a shared account we all contribute to. Once the busy couple downstairs moved out, we decided we'd do it ourselves (tiny town garden, so hardly onerous!). The outcome is that I do (and enjoy!) the gardening for the front garden, and the tenants downstairs do the back. The third, upstairs owner doesn't garden - he's a cleaner so probably doesn't want to clean the garden when he gets home!

The (ancient) lease grants access rights to all three leaseholders (for access to drains/plumbing and I suppose meter reading too) but no mention of the garden is made. My old deeds just outline the footprint of my flat.

I want to sell my flat in the next 1-2 years I'd really like to overhaul the front garden to make it more manageable and more attractive for selling and to add value. But I don't own it.

So my question is Would it increase the value of my flat if I were to buy the other two owners out of the use of the front garden?
Snag 1: I don't own the freehold. So do we (one or all) leaseholders have to own the freehold before I can buy someone else out? Can I buy "exclusive access", or something similar, for the front garden without the freehold?
Snag 2: Am I stirring up an expensive hornet's nest if I try to buy the freehold? The previous owners tried to find out who the freeholders are and didn't get anywhere. Would finding the freeholder be costly? Would buying the freehold be costly? Time consuming?
Bonus ball: The owner upstairs inherited the flat and earns little, and we will need to pay soon for external redecoration and repair. So it could be that we help the upstairs owner to realise some of his home's value by buying his garden access so that he can contribute to the work (which he would choose to do).

in summary Can I and should I try to buy access? Or should I just revamp the garden?

Thanks, and sorry for rambling.