I hate to be boring but I like Premier Inn and, in my experience, they give you the option of a non-refundable price or, for a little more, a flexible booking where you can cancel up to 1.00 pm on the arrival date. With the flexible deal, if you book for several days but want to leave early, you can cancel the rest of your stay and get a refund. The 'good night guarantee' is real - at least it was for me with a genuine case of loud noise in a room next to the car park. I don't stand up for many companies these days because they are often pretty hopeless but I think Premier Inn do, usually, give good value. This looks like an honest mistake in booking and I sympathise; but I can also see the company's point of view. If it refunds the cheaper non-refundable deal, why would anyone pay more for flexibility?