An external fire stair is to be closed for 6 weeks during refurbishment of a courtyard, only 1 more fire escape will remain to serve the 1st and 2nd floor (total of under 60 tenants) where the 3rd 4th and 5th floor have multiple alternative fire escapes. If closure acceptable on this basis? Many thanks

Without a review of the Fire Risk Assessment there is no way of knowing if the premises is acceptable but as an alternative was considered for the 1st and 2nd floors it looks like the premises are not acceptable and the enforcing authority (local Fire & Rescue Service) should be informed.  

Without seeing the site the answer by default would be no. This issue should have been considered at the planning stage of the works and, as a significant change, the fire risk assessment reviewed. If this has not occurred then an offence has been committed.