I want to stabilise some pebble dashing on an 1865 two up two down. The walls are yellow London stocks with lime mortar that at some point have been pebble dashed over. Key is breathability.

The render for the dashing appears to be a very sandy mortar. Not sure if that had lime in but the brick mortar behind certainly did.

It's my understanding that the better bricks were used on the fronts of the houses and the rougher ones on the rear and chimneys. Pebble dashing was a good cover up.

Resin, acrylic, polymer types are seemingly not the thing to use.

Commercial products based on "silicate" look to be around £60/5L.

Could I use waterglass /sodium silicate sprayed on instead?

Someone else there has suggested good old Thompson's Water Seal which I'd discounted but apparently it's breathable!

I'll be spraying whatever on from a cordless sprayer.

You could try Stormdry. I have used it on porous brickwork and it has been very effective at preventing penetrating damp.

http://www.stormdry.com/questions/can-stormdry-waterproof-render-roughcast-or-pebbledash

Sodium silicate will bind it to a degree, but I'd steer clear on old brick and lime. It leaves sodium salts behind as it cures, and those come out as white bloom and can do more harm than good in a wall that's already handling salts and moisture. The conservation route is either potassium silicate (the base of the mineral paint systems) or an ethyl silicate consolidant, which deposits silica in the pores without any salt residue and stays breathable. Both work best sprayed at low pressure, flooding the surface until it stops taking it up.

Before buying anything I'd go over the whole lot with a knuckle or the handle of a screwdriver. Any hollow sounding areas have lost their bond and no consolidant will hold them on; they want cutting out and patching in a lime mix with a matching dash thrown on.

Stormdry and Thompson's are water repellents rather than consolidants. They'll stop the surface soaking up rain but won't glue loose pebbles back, and if there are hairline cracks water can still get in behind and then dries out more slowly. If you do use one, do it after the consolidant and after any cracks are dealt with.

Mask the windows well, as silicates etch glass and are hard to get off, and pick a dry, mild spell with no frost forecast. Do a test patch round the back first.

It looks very similar to what I have, so if you want to come and practice on my place, feel free.

Free board while till the job is done.