Fill in holes left by gas heater flue

[plasticsign]

Hi guys, does anyone have any advice on how to go about filling and and repairing these holes left by gas heaters?
They're solid stone walls about 1ft thick, one is white tricky to get to from outside the other 2 are accesible. Looks like the flue has been cemented in place - not sure if it's best to leave the flue in place and patch up somehow or try and take it out (I have no idea what I'm doing!)
Thanks in advance

 

[JohnD]

That sort of thing is often pushed in loose, and mortared at each end.

In which case you can chip it loose at each end, pull it out, clean the hole and pack with mortar and stone pieces.

At the outside end, press the biggest pieces you can into the fresh mortar to try to match the wall.

 

[opps]

Personally, I would be inclined to vacuum the area, then bist it with water, then spray in expanding foam, and face it off with a proper filler.