I am of the opinion that a door frame stop on a fire door cannot compliantly reduce a threshold gap issue. If this is correct - is there any legislation that I can quote from pls?

Check the doors burn certificate, BS 8214 requires that fire doors must be installed in accordance with the test evidence.

But i doubt that you will find one that states "fit a door stop to reduce threshold gap"

ADB states "Fire doors should be installed in accordance with the manufacturer’s installation instructions and relevant product standards."

BM Trada stated "The doorset must be installed exactly as tested and detailed in the manufacturer’s installation instructions."