Is it a legal requirement to have a vision panel in fire doors.  A commercial multi occupied building with unit doors onto communal corridors which is a fire escape route - can they be solid or need vision panel?

Approved Document B to the Building Regulations only requires them as follows:

 

Vision panels are needed where doors on escape routes sub-divide corridors, or where any doors are hung to swing both ways. Note also the provision in Approved Document M Access to and Use of buildings, concerning vision panels in doors across accessible corridors.