Which form do I complete with regards to fence boundaries on a new build development

I was advised verbally in March 2024 and in writing "your private garden" in April 2024 by Sage Housing confirming that my upstairs maisonette had access to the rear garden.  I am an affordable housing tenant with disabilities and recovering from radiotherapy so sitting in a garden in the warm weather is important to me.  The original development plan attached had my plot number and downstairs plot number printed on a plan of the rear garden, with a red line around the actual property.  The developer built the downstairs master bedroom with french doors onto the garden, so they are not happy due to security.  To resolve this Sage Housing and the developer have decided that they will build a fence so only downstairs have access to the garden and upstairs just have access to bin storage.  All the 12 maisonette properties have been built like this, with large gardens, but the 2 x sheds/bin storage areas have different layouts.  All the maisonettes upstairs and downstairs have the same floor space and were listed at the same rental price.  As I am making a complaint to the Housing Ombudsman Service and will be getting legal advice, I am really confused which form I am meant to request and which plans I will need.  On checking the property description it just states where the land is situated, so I also want to make sure that I am not paying for just an outline of the land.
Thank you for anyone's help with this as I am very stressed about it.

Claire - it's all a Q of how it has been registered and often with a new build you will need the register plus a copy of the Transfer of part or Lease depending on how the ownership has been dealt with 
You can check the register online, once registered, and then apply by post for a copy of the transfer or lease as appropriate
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