I live I a converted building containg 3 flats. One of the leaseholders advised he has had a fire door fitted but it appears to be a second-hand door with a piece added (around 4 inches) to the bottom of the door! It us very obvious as the top/original door is painted with the addition being bare wood. I am assuming this cannot be done?

I agree with you it does seem suspicious but you need  to check the door more closely and if you study https://www.asdma.com/knowledge-centre/ and select to appropriate download, I can see a couple that could be of help to you. 

Hi Bernie, Any modifications and repairs made to fire doors should be done competently and I would advise careful investigation of the modification/repair work to evaluate its suitability.