Having recently undertaken a course, though informative on the should not's but not so on the can do's. So what can we do to reduce the gaps on the sides of fire doors, I seem to get conflicting answers on the right approach. Adding veneer/ liping simply on top bonded with glue and tacks to the door, I'm sure I read somewhere was a definite no,no ? and instead remove the existing edge and replace with a thicker one ? If that's correct is there a maximum point in which we go to before it's time to replace completely? Can we add fire strips may be? Thanks for any help 

You are correct there are issues surrounding lipping min & max thicknesses, type of adhesive and density of timber.  A good maintenance training course will deal with these issues.

Also, if the door is in scope of a 'third party product certification scheme' such work may cause the certification to be void.  Mind, it already is when the gaps are too large!

Best practice would be to remove the door leaf and door frame, adjust the door frame to suit the door leaf and re-install the door assembly compliant in all dimensional requirements including all gaps and alignment.

Hi, I am looking at some steel flat fire doors in a high rise that have excessive perimeter gaps. I don't have the manufacturer's original specification. How can the gaps be reduced? Is the only option replacement?

Contact the Door & Hardware Federation for advice  https://www.dhfonline.org.uk/