I had the same issue at easter. Booked an Audi TT and (after much complaint) got a VW Eos. Again the staff claimed ignorance of the guarantee, but I had fortunately printed off the page of the website that states it in black & white. I sent a complaint to Hertz UK marketing manager, not asking for compensation (as the Eos cost the same) but asking how I could ever trust their guarantee again. I recieved quite a good reply, apologising and agreeing that they were at fault due to overbooking. They also gave me the regional manager's contact details and offered that if I do book a guaranteed car again I can email him and he will personally ensure that I get the car I reserve.