The land registry detail will show you if there is any charge against the property including mortgages and who holds the charge. I think it will also show who the owner of the flat is, not the person who hold the freehold of the block
You need to check the leasehold title and the freehold title - the leasehold title will show the person who owns the flat and if they have a Mortgage. The freehold title will show the freeholders.