Tile backer board - plaster has come away from wall exposing brick/block work

Tile backer is a good idea - To make sure it's flat against the brickwork I'd use some "Sticks like shit" to take up any irregulrarities (I'd actually use drywall adhesive but for a small job it's probably not worth it) and screws every 300mm. If you're talking a lot of wall - say an entire 2m long wall or similar then use "dry wall adhesive" - mix it up with a mixer on an electric drill to a stiff creamy consistency and dollop handful size gobs onto the backer board every 300mm then just stic it onto the block/brick - fantastic stuff!

Yes you can use Aquaseal WaterProofing compound it is all ready to use liquid and easy to apply some benefits of this products : - Can be used on most common building surfaces - Ready to use, no diluting necessary - Easy to apply by brush - Non-hazardous, solvent free - Can be tiled over after 12 hours also can be easier use to diferit area such - Showers, wet rooms and wet zones in bathrooms - As a Primer on most common building surfaces - Balconies and partially covered external areas And can be applied to all common building substrates such as plasterboard, MDF, plywood cementitious boards, blockwork, concrete & speciality wet room boards