have had an old gas fire and its back boiler removed and replaced with a combi boiler what is the best way to insulate and fill the hole left from these

Rockwool or expanding foam can be used to fill the majority of the void, Once rockwool is tight and compact you may be able to use some rigid insulation (kingspan or celotex) to block the cavity. Once this is complete, depending on what finish you would like you can either plasterboard and skim or fix mdf panel to the front (smooth off edges of mdf panel for a less obtrusive appearance). Good luck!