Thanks All
Unfortunately can't avoid these premises as they are existing and known to the FRA.
Legal arrangements are fine as long as individual participants remain in situ and the agreement is formalised in writing. This agreement should/must also be detailed in the Fire Risk Assessments, and sited by the FRA.
However should one occupier change, a notice served under the Party Wall Act will allow the new occupier to brick up the alternative MOE, rendering the remaining existing occupiers MOE inadequate, and there is nothing that can stop it. I have seen this happen more than once.
Also, notwithstanding agreements between occupiers for alternative MOE, problems always crop up over testing/drills, communication and cooperation, and maintenance. Would anyone argue that these openings between different premises are common areas, and therefore the responsibility of the landlords?