“Some properties, mainly flats and apartments, have boiler flues which cannot be inspected because they are hidden behind walls or ceilings. The boiler flues that this information relates to are connected to room-sealed fan assisted boilers only. If you live in one of these homes then please read on for more information so you know what to do.”

http://www.gassaferegister.co.uk/advice/flues_in_voids.aspx

If you are a tenant, it is the responsibility of your landlord to ensure that the boiler is checked every year and the flue is able to be examined.