A very common scenario.
I think that the Landlord must do all he can to protect the common areas and the front doors should be seen as part of the common areas and not the tenants or lessees property.
However the change to the Law in 2006 were not forseen by landlords and conveyancing solicitors and many leases do not specifiy any control of this important element.
I guess really that changes of the front doors may also have been subject to the Building Regulations and an application should have been made? Have at regualtion 3 of the Building Regualtions.
http://www.communities.gov.uk/documents/planningandbuilding/pdf/explanatorybooklet.pdfThere is a problem here and I speculate that under the Fire safety order the tenant could also be considered to be a person having control as their actions have detracted from the fire safety within the common areas. If the lease says the door belongs to the lessee then IMO they are a Person Having Control. If it belongs to the Landlord the lessee is just a relevant person.
If the lease is unclear, and if the door has been changed without authorisation or Building Regulations approval, then I think the Landlord is in a strong position under the Order to lean heavily on the person who changed the door. I think the landlord should change the door and then persue the tenant for the cost in doing so.
If the lease says the door belongs to the lessee then they are a person having control and full sanction of the Fire safety Order could be applied to them.
The landlord must make sure in future for any further sales that the demarcation line is clear and the doors are controlled. The Landlord has to be able to show due diligence in his efforts to resolve this problem and if he can show he is doing all he can, subject to the due processes of the Law, then he should be covered.