[Can thumb turn mortice locks be used instead of FB locks on a services cupboard.]

Thanks for the feedback. I was not aware of any defined requirement to fit FB locks either but a recent Fire Risk Assessment Report described a requirement to fit FB locks. Any suggestions or links to regulations I could use to challenge their statement. 

[Can thumb turn mortice locks be used instead of FB locks on a services cupboard.]

The double leaf fire doors to the services cupboard  in the communal corridor of a residential block of flats displays the mandatory blue "Fire door keep locked".

Both door leaves are fitted with concealed door closers. One leaf is secured using a top and bottom door bolt fitted inside the cupboard, and the other leaf is secured to its mate with a FB mortice lock. 

Residents need to access the cupboard to read their gas meter and access the water stopcocks. The FB key to unlock the door is made available in an unsecured lockbox located adjacent to the services cupboard.

The security of any door secured with a FB lock is illusionary given the keys are available to anyone, and in this situation the key is accessible to anyone in the area. 

On the basis the requirement is to lock the services cupboard door, can this be achieved using a thumb turn mortice lock instead of the FB keyed lock or do regulations require it to be a FB key?