But I should rip the current ones off and tidy as best I can behind it? (Just so the worktop on one end of the return isn't a centimetre out compared to the other?)
Yeah take the old ones off first, you'll probs rip the wall to bits as thats what has happened every time I have took tiles off, least it gives you time to get it skimmed or touched up if it does.
Yes I would wait personally