We have had an independent fire risk assessment undertaken at my small offices and warehouse.

They have asked us to change all existing hinges on our fire doors and add a third, central one, on all fire doors.

These doors have been as is for 30+ years.

Is this correct?

And would it be a recommendation or a legal requirement?

Fire doors usually have three steel hinges to prevent the door twisting in a fire and the FR assessor has checked the fire doors, coming to a decision that to meet the required fire standard it requires three hinges. There is no particular regulation that requires three hinges except article 9 of The Regulatory Reform (Fire Safety) Order 2005 state you are required to conduct a suitable and sufficient assessment of the fire risks and it is necessary to implement these finding.

The fact that it has not been noticed for 30 years, is irrelevant, it has been noted now, it is just you have been lucky that these door were not called upon. If things were to go pear shaped and these fire doors are brought into question then you could find yourself in a difficult position.