Wedi Board fixing - advice needed

[dashithetiler]

I'm hoping someone here can give me some advice!

I'm re-tiling my shower after needing to rip out the previous dodgy plywood backing which failed (right side in the photo) and plasterboard (left side) I'm planning on installing Wedi board as a backing board solution.

For the right side onto the wooden frame i'm going to use Wedi 12.5mm board using the screw/washer method to the existing timber.

The left side onto the breeze block. What is my best installation method? I need to achieve a depth of around 35mm to match the non-tiled wall to the left and also to sit in the right place above the tile-upstand of the shower tray.

I'm thinking of using the 30mm board with the Full surface or Spot bonding (with dowelling) method. Although as the the wall is pretty straight - would adding 25mm battens and 10mm Wedi board work here? Then using the screw/washer method

 

[Ben-gee]

Either method would work. I would use the thicker board and use tile adhesive to attach it , then fix with a few washers once it’s all set- allthough tbh I don’t think they’re really needed.
The advantage of this method over battens is that it is quicker and easier to get a perfectly flat and plumb surface.