You can do it! Don't use a readymix adhesive - use a bagged adhesive like this https://www.screwfix.com/p/mapei-rapid-set-adhesive-grey-20kg/29634 Don't mix it too sloppy. DO butter the backs of the tiles as well if using a stiff mix. This stuff skins quite quickly and if you push tiles onto skinning adhesive they won't get a good grab - especially if the backs of the tiles are dusty. This sort of adhesive will easily pack out your 10mm concave. Get the first row on absolutely straight and flat, using a straight edge and let them go off so they are firm. You now have a row of tiles with a straight front edge which you can build off. If you use a rapidset you can do small areas and let some tiles go off and then carry on - just make sure you wash your tools/bucket down in between Get that first row perfect. You don't need to pay a plasterer to flatten the wall first. It is a bit trickier tiling onto a non-perfectly flat surface, but take your time, check with a straight edge, and it'll be OK.