Thanks for the replies. I'm a union safety rep and have just done a week Fire Assessor training and have begun a risk assessment of the building where I work. So sorry for the lack of clarity! The building in question was constructed in the 1980s. It is a three story office building with workshops attached. Water company. My concern is the first floor area which is used by call centre and admin staff.
The maximum occupancy of the floor is 300. It is an open plan office in an H shape. There are two protected staircases as escape routes. Each escape route can be accessed by two fire doors - one on each side of the H.
So if I exclude the largest staircase this leaves a rear staircase accessed by two 1200mm doors both leading to a narrower staircase - 1100mm.
Now when I looked at DCLG I thought that would mean it could accommodate 200. I asked the facilities manager to clarify and he said he has a different view and referred to Building Regulations and the fact that people were familiar with the building and awake. So he is referring to BS9999. I think when I referred to Building Standards I was referring to Approved Document B.
This is a steep learning curve for me so thanks for your help.