I've got someone coming to repoint the back of my house in spring (he's done quite a bit of work for me and whilst a little clumsy/messy definitely not a snake oil salesman) he's recommended applying this as a boots and braces type thing (north facing wall).
That’s what I thought initially, then looked into it and it’s this kind of stuff which doesn’t seem too bad from reviews.
https://www.wickes.co.uk/Stormdry-Masonry-Water-Repellent---3L/p/133883