In the US Groupon guarantees you'll be satisfied with your experience. If you didn't like the restaurant then tell Groupon and get your £18 back. This restaurant sounds like they did a very bad job of designing and executing their Groupon and it hopefully will end up costing them money and reputation.
Unless that policy isn't in place for Groupon in the UK?
Unless that policy isn't in place for Groupon in the UK?
http://www.groupon.com/groupon-promise