I'm planning to tile the walls between the base and wall cupboards in the kitchen In order to hide a gap (approx 20mm) between the rear of the new worktop and the wall (it's a long story!) I need to pack the space between the plaster and the tiles with something. I could use a huge amount of adhesive, but this will probably lead to an uneven finish. Any bright ideas on how to achieve this. (Note that the wall is not even, so whatever I use, I'll still have to fiddle to get this right.)