I live in a ground floor flat and attached is another flat where their kitchen is on the ground floor (behind my bedroom wall) with their living accommodation upstairs. There is a connecting door which connects the 2 flats just outside our bathroom and access corridor and at the bottom of the stairs of next door flat opposite their kitchen entrance. This has been designated as a Fire Door by our Landlord but it is not 'fit for purpose' in my opinion. It is not properly sealed for smoke ingress in to my flat (no prevention) as I can see lights on next door through the door frame and anyway it is always kept permanently locked. What I need is a legally backed argument where Regulation and Requirement is present to hand to my Landlord.
Would this adjoining aperture be better sealed permanently (bricked up) or what?
I have always been of the opinion that Fire Doors should never be locked and anyway, this particular fire door joins the 2 flats and if left open would negate security between the 2 flats.
Can you help me please?

I'd need to see it for a definitive opinion, but it doesn't sound satisfactory. Sounds like conditions may be contrary to the Housing Act 2004 - you should contact the housing officers in the Environmental Health Department to assess the flats under the The Housing Health and Safety Rating System 

If it is a fire door in a common area to provide fire and smoke separation then under Article 17 of the Regulatory Reform (Fire Safety) Order 2005 the landlord / building operator has a duty to ensure that the fire door is ‘subject to a suitable system of maintenance and maintained in an efficient state, in efficient working order and in good repair’. This means the ‘Responsible Person’ under the Order has a duty to ensure the door will provide sufficient protection to contain the fire and smoke so as to ensure the safety of persons in or around the building in the event of a fire.