Hi I live in a block of 4 flats with communal hall and stairs, the council have just posted a letter saying we must move these within 7 days due to it being a fire risk , is this a regulation now in place ? 

No, however their fire risk assessment under the Fire Safety Order 2005 must have decided that the mats were an undue fuel loading/fire risk (presumably arson as there aren't many other ignition risks in a flat common area) and to be removed.

 

Further to AB's reply another possible reason could be  The Regulatory Reform (Fire Safety) Order 2005  article 14 requires escape route to be clear, which I suspect is to prevent obstructions or tripping hazards, which could be a serous situation in an evacuation. In this case the Responsible Person could have considered the doormats to be a tripping hazard and wants them removed, but in many cases this situation is risk assessed, also in most homes in the country have their doormats are behind the front door not outside. 

...although the flats should be stay put and no one should be evacuating other than the flat on fire.

The other issue with mats sometimes is they are the thin edge of the wedge and because they are allowed all sorts of other things start creeping in the halls, you see shoes, decorations, prams, etc, etc and so many councils and HA's opt for the easier to manage total ban option than managed usage of these areas.