At the beginning of the year, the Health and Safety Executive  introduced new rules regarding gas flues fitted in void spaces or behind walls or ceilings. From January, Gas Safe engineers have the authority to declare a boiler "at risk" if they have no ability to inspect and access the entire flue. Then, with householders permission, they have the right to turn off the system and advise households not to use the boiler until inspection hatches have been put into place. These rules will apply mainly to flats and apartments and so Landlords who own such properties, which usually have room-sealed fan assisted boilers, need to be aware of the new regulations. It will be the responsibility of the Landlord to ensure that these measures are put into place and an annual gas safety certificate is carried out. Hatches must be fitted within 1.5m of a join in the flue and must be a minimum of 300mm square.
Quoted from http://www.onlineprnews.com/news/346449-1361176070-gas-safety-update-for-flue-in-voids-introduced-by-hse.html