Filling in a deep hole left by a cat flap in the wall

Very surprised you haven't been answered yet. I do hope everything is well and you still have your cat Ok, if your doing it yourself then little by little. Not knowing how long the tunnel is but lets say two feet, if you have access both sides and I'm assuming a standard cat flap then two bags of ballast and one of bag of cement will be OK. Make sure the mix is three equal parts of ballast to one of cement then fill the hole half way.(not too much water as your mix should be like a thick cauliflower cheese mix). If you have a DAMP COURSE where you are working then you will need to buy and insert this at the adjoining /appropriate level to your house. (realistically overlapping the original damp course by 150mm each side right and left). Leave for one day the work you have achieved to set then do the same again the next day and use a long rod to pack in any voids or air pockets to solidate the void. Don't worry about the excess that has filtered out as you can dislodge that as it will be soft enough to discard and smooth off with just a cement float

First I ask is it stone or brick if stone I would use lime Motar and build stone through and around the tunnel with nhl5 2:1 mix finish with 3:2point on surface point internal and external