I’ve not heard of concrete between joists but I have heard of sand between joists for acoustic insulation. Could be the previous owner/builders just swept building material between the joists?

Maybe some attempt at fire blocking?

It's very odd, as you can see from the pictures, not all the joists have it in between.

When I look at one picture I think one thing and then another picture another.

One picture make the “concrete” look like cellulose insulation. But since you describe as concrete I guess it’s hard and you would expect cellulose to be like paper.

On another picture I wonder if you have got fibreglass insulation with some building debris on top. Builders are devils for just sweeping everything under the floor and I suspect that’s what this is. It’s normal to not insulate some gaps between joists if there is a spotlight in the ceiling that might produce heat and so best practice says you don’t pack anything around that light fitting.

Looks just like my cavity wall insulation but it’s light and fluffy, like a mixture of wool and polystyrene bits. I’ll see if my brother knows, he’s a builder.

My brother said it looks like either insulation or builders rubble. But he’s just had a second look and said it’s rubble previous builder left it there, naughty boy. His words not mine!

Thanks all for your comments, just checking if I'd missed something obvious, last thing I want to do is make a huge mess when gettting rid of it.

looks like someone has just left it there to clean it up by someone else