This deal seems to keep finishing and then they start it again! Now valid for booking to 30th June - Says for UK only, but i've stayed at Sofia (Bulgaria), Prague and Helsinki hotels using the 241 deal!
Click go to deal button and use the search tool on left, make sure rate preference is set to 'exclusive 241'.
Not all hotels are included in the offer, but it is normally pretty good availability. When you search it'll show an 'average nightly rate', which will be 50% the standard rate. If the hotel is not doing the 241, it shows "The requested rate is not available." in red.