In an office building , can fire doors be locked at night when know one is at work or does the fact an exit says fire exit then it must remain day and night?

The reason is the building can be accessed 24hr a day by staff from another area , although no one would be working there at night and would have no reason to be in there at night.

The fire escape route goes from a corridor , through an office and into a carpark. The door gets locked to the office as security I'm presuming.

Once a week at night I check the fire alarm at night. 

 

The management say that they can take the signs down but due to travel distances they cannot . Its just the overnight position I'm not sure of.

If you can point me to the paragraph in RRO or BS I would be grateful.

Fire escapes need only to be available when the premises are occupied. As it is an office I am assuming you are talking about illuminated maintained fire exit signs and they could be replaced with standard exit signs if there is sufficient illumination or non maintained emergency lights/exit signs if not.