I really couldn't think of any other word to use other than grouting!!
Anyway. I'm about to polish up and treat some internal sandstone
flagstones, and all the edges (between each flagstone) needs
"re-grouting". What would be the recommended material to use for this?

I remember someone telling me to use a polymer based material as it is
much tougher than other materials such as sand and cement.

Thanks in advance

James