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sandy shite
Probably some on-site knocked up sand/cement mixture,
with insufficient cement by the sound of it.A non-slumping epoxy repair mortar would be ideal,
(apologies I no longer have ready access to such materials),
but no-one sells just a cupful!
Toolstation item 46209 is the kind of thing,
(don't know for certain it is non-slumping),
but at £25/4kg it makes a small diy job kinda pricy.
It's got a M10 plug in there at the moment, but the main problem is the brick has been filled badly with some sort of sandy shite. I get the feeling I need to dig all that out and fill it with something really solid, then drill fresh and probably use those anchor bolts.