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Firstly no, you are not overreacting.
I would ask for a break down of costs and for a copy of the receipts for the materials that the builder purchased.
I completely understand you don’t like confrontation, I don’t either but I would call a meeting with him and bullet point what you want to discuss in an email to him in advance.
If you can I would also get a couple of other quotes for the remaining room just to gauge how close the builder’s tiler’s rates are to your area’s average. It sounds expensive to me but I’m in South Yorkshire.