Short answer
To find ownership of a registered flat, order the Title Register for the flat's title. For most individual flats, this is commonly the leasehold title, although the building may also have a freehold title.
Why flats can show more than one title
A block of flats can have a freehold title for the building or land, plus separate leasehold titles for individual flats. This means an address search may show both freehold and leasehold options.
Which title should you choose?
If you want the registered owner of one specific flat, the leasehold Title Register is usually the starting point. If you want information about the wider building or land, the freehold title may also be relevant.
What the Title Register can show
The Title Register can include registered proprietor details, title number, tenure, property description, charges, restrictions and rights affecting the title where available.
Official source: https://www.gov.uk/get-information-about-property-and-land/search-the-register
What if the flat number is missing?
Use the full address as accurately as possible, including flat number, building name, street and postcode. If the address lookup is unclear, manual address entry or additional title checks may be needed.
Independent service note
LandRegistries.com is an independent document-ordering service and is not HM Land Registry, GOV.UK or any government department.