As a general question if a corridor is carpeted and this runs continuously under the fire doors should this be broken up with a threshold as a fire break ? 

I would think it depends on the carpet used, if it has passed the hot nut test, then not that critical but fitting FR threshold is a much better solution.

Looking at this from the fire door survey point of view.  The British Standard for timber-based fire door assemblies (BS 8214) states no requirement with regard to floor coverings at the threshold.

Installing a threshold strip to the floor is always preferable as it helps meet compliance with door leaf to floor gap requirements. BS 8214 and BS 9999 refer.