Hilton has launched a new promotion for its European hotels.
It is not valid for UK properties, but does cover hotels in Spain, France, Netherlands, Italy, Greece etc.
You need to book by May 20th but you can stay at any point during the rest of 2013. That said, there were plenty of dates blacked out when I checked the Hilton Sa Torre in Mallorca.
The link to book is here. The small print says that a one-night deposit is required at the time of booking, but a sample booking I tested was fully flexible up to 3 days before arrival.