I think it is definatly a PR stunt. I bet 90% of people using the vouchers would normally bought their alcohol from Tesco's/Siansbury's/Asda, so they are getting people in. The price of alcohol in a Thresher is about 35% more than in supermarkets anyway. It's cr@p that they will lose money or they would have simply not honored to offer or simply taken all the wine and champagne off the shelves and told customers they had sold out. If they even make �1 profit on every sale they have made millions. Plus people will also no doubt buy a packet of fags/chocolate bar which are not in the offer, and generate them more money.
It's in my opinion an incredibly clever marketing tool, 1 person in their PR department makes a pdf and e-mails it, costing lets say about �1, the revenue, interest and market share it has generated are phenomonal.
Mr Thresher is laughing all the way to the bank.