The gaps around fire doors installed a year ago are over the advised 2mm - 4mm so need reducing. This maybe able to be rectified with adjustment to the hinges but if not could someone please advise on the best method to close the gaps. Intumescent strips are already installed so can another stick on intumescent strip be added to either the door or frame?

I know you can buy fire door cores and build a fire door which includes fitting side lippings. To do this you are supplied with the core and instructions (a global assessment ) which provides the information how to achieve this and include the fitting of side lippings, dictating the sizes to use. So using a wider lippings may reduce the gap to the required gap. You would need to remove the existing side lippings and replace it with a new ones but I do not know how practical this is, consequently you would have to speak to a competent joiner.

I am not aware of any other methods that would be acceptable.

For more information check out https://www.egger.com/shop/en_GB/interior/product-detail/FIREDOORCORES there are others if you surf the web.