http://local.amazon.co.uk/Central-London -- Amazon's Deals site looks to be as bad as all the others. Full of the same deals - massages/facials at failing salons, dinner at low rated empty restaurants and so on. Often these offers are the same as what you get if you visit the venues' websites and look in the offers section.
I've used KGB Deals, Wowcher, Groupon & TimeOut Deals. I've found TO Deals to have the most unique London based offers. GroupOn is definitely the worst for spam that's hard to unsubscribe from.
Does anyone have other Deals sites recommendations?