We have business next door who use a route out of the back of their building across a roof and then into our building as a secondary fire route, up until now this has been working as our premises closes after theirs, now the tennats have changed and are working later than us. For security reasons we have locked this door whilst our premises are unoccupied, the new tenants are saying that we must now leave this door unlocked, meaning that in the event of the alarm sounding this door would remain unlocked  ( on magnalock ) leaving our building unsecure and the new neighbours access to our building.

Any thought or legislation that relate to this would be helpful

To be fair, this would not be acceptable for a building contructed now, but there are literally 1000s of such buildings with escape routes like this in London alone.  Current fire safety legislation came in during 2006 and not all buildings can fit the drawings in the guide books. So innovative legacy solutions such as over roof escape routes have to be accepted - if properly assessed and maintained

When I was a Fire Safety Inspecting Officer, I saw a  5 storey single staircase hotel which had an escape route out of a window, along a balcony and in throught a window which released on a maglock. The neighbour was a residential flat - and it was their bathroom!!!! The young lady who flat it was had a small buzzer in the bathroom so she had time to  protect her modesty if she were soaking in the bath! Sounds like a Carry On films doesnt it!!!!??

In this case you need to check as there may be a legal document drawn up (For the life of me I cant remember what they were called), but most agreements were a friendly hand-shake.  Alternatively, the lease may have conditions. Again, a simple read through would give you the answer

Article 22 of the Regulatory Reform (Fire Safety) Order 2005 says (paraphrasing) that Responsible Persons must cooperate and cordinate with each other. I doubt and local authority would take any action aganaist your company and would rather let the two parties settle the case in court. In fact I was called to plenty of such disputes by the person whose mean or escape had been blocked, and ended up making a voluntary agreement not to occupy the upper floor(s) of the building, or in some cases, serving an notice on them. That didnt go down so well!!!