wall preparation - Tile Advice

[Forestface]

Id like advice on preparing this wall. The old adhesive is proud by 5mm to 10mm in places from the wall. What would you do to prepare this for 200x500 ceramic tiles. only half the wall had been previously tiled and its in good condition but half is this

 

[eddcottee]

Can you overboard the whole lot with tile backer board? It's probably the easiest and best way of sorting those walls out. Not massively expensive either. I find Hardiebacker easy to use but others prefer some of the fully waterproof alternatives such as Wedi etc