I’ve been investigating the best way to patch and re-point sandstone. Apparently a big no-no is using normal cement mortar as it will actually damage the sandstone as it is impermeable to water and will actually force water into the sandstone causing sloughing, cracking etc. In the worst cases, the stone erodes leaving a matrix of cement mortar behind….like above.The answer is to use Lime Mortar which is porous and will allow water to move out of the stone.
Our stonework is mostly okay but there are patches which we need to investigate, the stone has spalled. is it a leak, weather or bad previous pointing. 
Found these guys and think this may do the trick.
Will report back!
