Home DIY

Hard to get scale from that picture but I've made some up to a couple of inches wide and hammered them in to support radiators where the wall has broken down. The problem you have with alternatives is that dust and mess. You need to clean it all out to some solid material and use pva to help bond it. Then use a filler like Fibarex or Murex and let them dry thoroughly, that could be days if that's a big hole. This is where smashing in a wooden plug (even a big one) can be useful because friction with the surrounding material helps to keep it in place and you can use filler/pva to help stick the wooden patch to the surrounding masonry rather than dealing with a big fresh patch of plaster.

If you decide to try another spot and just want to get those spots filled. Hard to do in one go but get rid of the loose material and use Fibarex or TX110 in layers over a day or 2. Even bonding would take a day or 2 to dry.

There are epoxy resin solutions for fixing bolts into holes, plenty of them. They fail very easily if there's unstable material and dust in the holes.

I cleaned the hole out and then used something like this (not this exact one though) for the same issue before. https://www.amazon.co.uk/fischer-539461-Injection-Resin-Clear/dp/B07W9LZFZ5?psc=1

You might also get away with using wetnfix with deeper plugs so that you get to some solid wall behind (EDIT - seems unlikely given your later post).

But if it's for a survey and being sold why not get something cheap and light?

I used to live in a flat that had that stuff going on in the walls. I swear it’s like soil.