Not an issue from a handling perspective but you may find the ride a little harder as the thinner tyre is very slightly less "bouncy". They should have replaced like-for-like, or at least asked you if it was ok not to. I suspect that they didn't have a 42 in stock as they are less common. You say that they fixed the puncture, hmm, if you were paying for a new tyre then I would have thought they would have given you a new tube as well. Not a major issue but a new tube is pretty cheap tbh and not much more than the cost of fixing the puncture I would have thought. What brand of tyre did they fit on the back ? Others my have their view on the best brands but I have always fitted Schwalbe Marathon Plus tyres to my bike that I use for commuting into town daily, year round and I'm tempting fate but I cannot remember the last time I had a puncture. They're about ?20 each.