Shared garden/freehold

[LCeles3]

I own the freehold of a semi-detached house in London,
I live in the flat upstairs.
My neighbour downtairs has leashold and the use of the rear garden.
Does anybody know if we would be able to put a fence up the middle of the garden splitting it into two making half for us and half for upstairs?
As freeholder I have to ask their permission? Thay must agree to do that?

what does the lease say, does she have exclusive right to the garden? in my last home, it was our private garden, no way would the freeholder be able to put a fence up!

Check the lease - I am leaseholder of a downstairs flat and the lease clearly marks the garden as all mine.

Even if your downstairs neighbour just has 'use of' the garden I still don't think you can unilaterally take away use of half the garden.