Our office has 9 fire exits, for up to 50 users (including visitors etc). The alarm system on one of the doors is not working. Can we temporarily lock this door and no longer use it as a fire door until we can resolve the alarm issue? obviously with clearly updated signage on the door and on all notice boards and with explicit consultation with the employees and visitors. The next available door is within meters, through one unlocked door, if unavailable (i.e. if that's where the fire is) another door is available about 5 meters away in the other direction.

Without seeing I cannot be 100% but you sound like you will have more than enough exits and width for your occupancy and no travel distance issues.