On 30/05/2021 17:48, NY wrote:
Our house has a patio of stone slabs (slightly irregular size, shape and
thickness - not rectangular concrete slabs). They seem to have been laid
on bare earth, as far as I can see from looking into the gaps, without
levering up a slab to look.

Some of the mortar "grouting" between the slabs has cracked and
disintegrated, so I'm trying to re-mortar the broken bits.

Have you ruled out joint compound?

I've just laid a new patio and i'm deciding what to do as the jointing
compound is expensive, but if it outlives mortar it might be worth it.