FWIW, a chargeback is the only language lastminute understand. I booked a top secret hotel in NYC. Two nights came to about £360. It was advertised as a "21% discount". Once the hotel was revealed, I compared the "top secret" rate to the non-cancelable rate offered by the hotel. The "top secret" rate was slightly more expensive than the non-cancelable rate. It did represent a 21% discount on the cancellable rate offered by Lastminute for an opaque booking, but that is irrelevant: a top secret room is non-cancellable, so the advertised 21% discount for booking an unknown hotel should be by reference to a non-cancellable rate.

I asked lastminute to credit me the difference between what I'd been charged, and what a 21% discount would actually represent. They refused to. I asked my credit card company for a chargeback of this amount (about £65). Beautifully, they said "no - it's blatant mis-selling" and recovered the full £360 charge. Net result was that I had two free nights in NYC, courtesy of Lastminute's daft approach. I also got to keep the £30-odd cashback from TCB for the booking.