As Much as it grieves me to agree with Kevin and Carlita (Ref
http://www.fantasygirl.co.uk/)..
I agree impregnation the ONLY way to go with sandstone. Although I would tend to use a water based one due to waiting for the british climate drying it out enough to use a "solvent" based one. (But that s my personal choice)
Buy from Kevin then you will have the back up and advice for the product you are using.