We are a volunteer charity and own a oblong brick building. On one short end of the building there is an exterior fire door. It is metal in structure with the correct opening facilities.  The other door is at the front of the building constructed of wood. At present the fire door is left wide open and the public use it as an access to the building.  Could you give some guidance if there are any infringements being incurred such as should the fire door be alarmed ?  Within the building there are internal fire doors, one open, one closed.  My main concern is the open exterior door but also the security and our obligations to public safety from all types of occurrences such as (intruders, thieves etc.).

All doors used for means of escape should be easily opened, without the need of a key, in the direction of escape. There are a number of ways to achieve this and dependant on the one you choose will dictate the precautions you have to adopt, movement from the outside to the inside can be as secure as you choose.

How long before a shop closes can you closes a back door that is also a fire exit.

When the last person securing the premises leaves the premises but if it is easily opened from the inside without the need for a key then it can be closed at any time.