Converting attic space above top floor apartment

Hi,

I own a top floor apartment in a small block (12 apartments above 2 retail units). There is a small hatch to the attic space above my apartment. The attic space could be converted as there is good head height up there.
I am trying to find out who owns this space and as I am the only apartment with access can I use it.
The OMC have no records to indicate who owns it. The builder is long since gone (property built in 1994) as it the solicitor that acted for them.

I am not sure where to go to ask. Is there a particular title deeds or plans that I need to consult and if so who should have them? I want to approach the other members with a view to use/convert the space but want confirmation I am entitled to do so first.

Any advice on where to find this out would be appreciated.