I have had conflicting advice regarding fitting locks to an office door in a multi-tenanted commercial building. The door opens onto a common-area stairwell which is the primary escape route. 

I have been told that I must have a BS8621 rim-lock so that exit is always possible without the need for a key. This seems fair enough and I accept that. 
However, for security, I want a good mortice lock in addition. This needs to be a BS3621 (keyed from both sides) so that an intruder who may have gained access through a window can't then carry his loot out through the door.

I have been told that, if I have this mortice lock, then it must have a thumb-turn on the inside because it's on the escape route, but I have read elsewhere that additional locks may be used whilst the property is unoccupied and fixed unlocked when the premises is occupied (remember this is a commercial property not a dwelling).

I am also of the understanding that the door lock should be EN179 compliant (ie opened from inside using a handle) and that the door should be "operable without requiring people to manipulate more than one mechanism" (Approved Document B para:5.7 iv). So, if I was to have a thumb-turn on the second lock, then the door would no longer meet Building Regs as there would be two locks to manipulate.

Am I right in my understanding that, whilst unoccupied, I can secure the door any way I choose? And, if so, where can I find this written?

Many thanks for any input.

I think I've found the answer to my own question: 

Approved Document B1 para:5.9 b.

In non-residential buildings (purpose groups 3 to 7), some final exit doors feature security locks that are used only when the building is empty, Such locks may be appropriate, but management procedures must emphasise their safe use.

As you have found out. If your procedure for ensuring doors are unlocked from the out of hours fastenings falls short you will be breaking the legislation, whilst it works you are fine.