Cheers for the replies guys
So if I use a trowel with 10mm x 10mm notches with a 10mm spacing for the floor and wall then I wont go far wrong?
How much of the previous tile residue do I need to remove? At the moment its stripped back but you can see some areas have a smidge of the previous adhesive left. I'd say its around 0.5mm thick or so? Worth stripping that off too?