I don't think you are being unreasonable, they could have alerted you to what their proposed remedy was- especially if they were not going to replace like with like. I'm assuming the 80mm is overlaps the counter and is not sitting on it's edge. It's not unusual to see a splash back fitted on edge, around the counter top but the solution your builder has used is a bodge, designed to conceal a mistake- it's an obvious flaw, liable to attract dirt and grime and effectively reduces the useable space of your counter top. You have to ask yourself the question: "would this solution be acceptable in a brand new fit-out- if I were starting from scratch" . The answer is a resounding "no!". Third time lucky, maybe?