Thanks for your question.
I hope I can be of assistance.
On the face of it you cannot sell part of the lease.
You could potentially grant a sub-lease of the garden land. You may need consent of the freeholder. I would need to see the terms of the lease to comment further.
Not easy to advise without the title document and lease. Do you have electronic copies of the title and lease which you can attach to a mail to me?
Thanks. Then I can take a look.
You can buy a copy from the land registry, for title and lease should be less than a tenner.
You need to establish that you have title to the land you want to dispose of.
You then need to check if you can grant a sub lease and need the freeholder's consent.