iolo wrote:

I have concrete, of course. And I believe one can get hold of
"plasticiser" to mix with it. But I am not sure if that's the solution
and if it is I'm not sure what proportions to mix it in.

There is a type of cement, called "hydraulic cement" which expands
slightly when it cures, and is what plumbers have used to plug up drains
and voids in underwater concrete.