I have lived in a retirement development for the past ten years and have always had a small plastic ornament on our flat door. A number of neighbours have something similar - such as a knocker/small plastic ornament. We've been told to remove them as they are a fire hazzard. All the doors are fire resistant doors and I wondered whether a knocker / ornament would hinder the fire door from doing its job and if there is a specific fire safety regulation about this kind of thing.

Regulations are not so specific as to deal with a door knocker/ornament in terms.  However, the fire doors must be maintained as adequate to provide sufficient fire and smoke protection to keep relevant persons safe from the effects of fire.  Whether or not the knocker/ornament is suitable for fitting to a fire door would be a matter for a fire door specialist.