Waterproofing spray for houses, your experiences and opinions please

[Bewilderbeast]

We have just had out house re-roofed and pointed. It's a yorkshire sandstone house and is over 100 years old so some of the sandstone is starting to degrade and the surfaces of some stones are crumbling.

Our builder has suggested a waterproofing spray to protect it but I have heard and read mixed reviews of this procedure.

So:

Have you had this done to your house?
If so why?
if not why not?
If you did was it worth it?

Any partners of builders who know a thing or two about this process please feel free to stick your oar in also.

I have limited time to make this decision as the scaffolding will only be up for another week. I'd be grateful for any help and advice.

I thought the whole point of an old stone house was that it needed to breathe

all my instincts are screaming "no! no! no!" but I don't actually know. Can you speak to English Heritage or someone who would actually know?

The need to breathe bit does indeed worry me. Particularly as I have come across a telegraph article which talks about silicone sealing your house being like wearing waterproof clothes and doing a physical activity.