Thank you all for your input. Very much appreciated!
The walls have been skimmed, lovely and flat so no excuses!
So 10mm square notch to spread on the wall should be ok, and then back skim the back of the tiles just to fill the profile. Is that just to build the sunken areas level with the rest, or should the skim be over the back of the whole tile?
Sorry to ask more, but when putting the big tile on the wall how much pressure should I put on it to bed it in. If spreading 10mm on the walls, should this be pushed down to about 5mm or more? I don't need a rubber hammer for it do I !!!!
Thanks for your time and help. Si