Block a flue but retain fireplace openeing?

Hi rob Lee The best way to do this is to use a cement fibre board available from all good builders merchants and diy stores . I would recommend 6mm thick , cut to size with either a hand saw or jig saw and placed up inside the opening . Once a reasonably good fit has been obtained then secure it there with either a silicone or sand cement fillet . Most importantly tho this must be vented .A plastic hit and miss vent fixed to to fibreboard is ideal for this ! Cement fibreboard is resilient , moisture proof can be over boarded or just painted . The venting is essential to promote an adjustable amount of airflow through the chimney to prevent damp issues .. Hope this helps !