We are currently inspecting some fire doors in new and existing buildings, the question is that in some legislation I have read that there is a requirement to replace 100% of the fire doors if a certain percentage require replacing, but for the life of me I can not remember where.  Can anyone else recall this or can I just carry on and replace individual doors which is what the owners want.

I am not aware of any such guidance but the current guidance is clear Fire safety in purpose-built blocks of flats, if a new door has to be fitted it must be FD30s fire door to the current standards and fitted with an approved self closer. If an existing front door is a well fitting, of substantial construction with an approved self closer then it can be accepted also I would require all door furniture meets the current standards. 

Consequently I would think to replace individual doors as necessary depending on door inspections.