Got treated to a Groupon deal lunch meal at a really posh Chinese in Cardiff Bay. Meal was a bit like the curate's egg, but on the whole, a positive experience. Now, the whole deal had been covered by our Groupon voucher. We also had a lemonade, a diet coke and a cup of coffee, which I knew we'd have to pay extra for. The bill came. £9.30. Wife took the bill and notices a charge of £3.30 (optional tax) had been added @10%. She asked about this and was told that it was the optional service charge.........but the 10% had been calculated against the cost of the whole hypothetical meal before the Groupon discount, not the price of the Groupon deal. We declined to pay.