I have 2 rows of 5 tiles to line the sink behind the tap. I was told at the tile shop that if the wall was 100% flat and they will have the weight taken up the sink and the first row then silicone sealant would be fine, then just seal the edges after.
Should I sand the painted wall above the sink first?

U can get a small tub of ceramic tile adhesive at a big box store. After tile sets grout.

If gloss paint, rough up surface with sand paper first.

Chuck

Aah yes I understand that, but right now I have silicone and tiles and am snowed in my flat, is it fine to just use silicone sealant to stick them up, and grout them? I don't see any probs other than holding them in place while it sets - but they will have they're weight on the sink, so just some tape!

That is not an accepted method to put down tiles. You can do it but it will not last as long as a proper job. Silicone is flexible and grout is rigid.