[Closed] Sandstone repairs

I have had few quotes to repair a couple of bits of sandstone on the front of our flat. One is above a bay window it has started to disintegrate and will need to be built up in layers to match the rest of the stonework and the other is above the entrance to the flats. It's a communal repair and the factor had some quotes. The cheapest quote says they will build up the layers with cement mortar and the other 2 say they will use lithomix. Have done a quick google search and found that cement mortar may not be the best way to repair this? Anyone know what the best option is here? The difference between the quotes is an extra £50 per flat so I'd rather spend more if the job was done properly.