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The problem with a modern material like Toupret (or cement) is that it has a very different porosity (I guessing almost zero) compared with sandstone / limestone.
This can mean that water gathers in unexpected places, and can damage the remaining stone though dissolving / freeze-thaw, or other water action.
NotThamesWater
hvsds
I used Toupret Murex on a sandstone sill with decent enough results.