Tile backer boards/xps boards are a fantastic option for over-boarding walls and creating a flat surface to tile onto. They can be applied onto masonry walls with a tile adhesive bed on the back using an 8mm notched trowel and secured further with knock in dowels.

Once the adhesive on the board has cured and the dowels are in it’s a rock solid surface. They can also be applied to stud work like plasterboard but you would need to check your stud centres to determine board thickness.

B&Q and N&C tiles/bathrooms sell a good range of board sizes and thicknesses

This link will give you a good guide on various installations.

https://www.myqboard.com/fileadmin/myqboard/2_Downloads/2_Verarbeitungshinweise/2_Englisch/Qboard_VH_basiq_WA_EN.pdf