Pointing slate fronted listed building

[clarkstv5]

I live in a slate fronted listed building and in a number of places the cement has fallen out leaving gaps under the overlaying slates. In windy conditions this then tends to blow the slates off the house. Two questions, first how should I fill the gaps (on a vertical wall - having trouble stopping the new cenemnt from just falleing stright down)? Second how do I clean up the mess I've made on the slates from from my first attempt (someone has suggested brick acid - worried this might damage the slates)?

[welsh brickie]

dont use sand and cement.Use clear silicone mastic.
To clear mortar stains use brick acid,this will not damage the slates.