G
Gazzer
Minnie those walls look a little, er rough shall we say, would you be able to skim up them with a little bit of adhesive? from the right of the crack in the wall to the left it looks like about 3mm difference, doing it now will save you time later. The crack itself? is it stable? is the plaster loose because it usually is around a crack like that. You could buy a couple of sheets of mosaics and use them on the shelf ledge, that might help you instead of trying to match up grout lines. Don't forget to prime the walls first with an acrylic bond. However you do that area over the sink you might have to split the tiles and that will not look good if you cant get the silicone to cover the joint. I would try for a mosaic band over the sink as well.
Or maybe a large tile in a contrasting colour to form a solid one piece cill ?