Thanks very much for the sound advice.
Not too sure if I can get those adhesives here in Belgium. I have done a quick google and see that they are modified cement based adhesives. Although they mention they they are good for low-porosity tiles, mine are pretty poros on the untreated backs...
Just about to embark on tiling a large splashback in the newly built kitchen onto plasterboard. the tiles are hand made moroccan 10cm by 10cm.
The backs of the tiles are uneven and when face down slope to the middle, like a pyramid (will attach pictures).
Should I use large spacers...