This is a great little shop in the heart of Hampstead with great bargains to be found. Check out the basement, where there are loads of lower priced items and sale stuff.
Of all the designer second hand stores I've been to, this is not overpriced and I've never come out empty handed so far. What is good as well is that there aren't just 'Model' size clothes, it is good for everyone.
If you have stuff to sell - they will only take a minimum of 3 items, so if they only like one of what you brought it's not accepted on its own.
The staff is very friendly and helpful and not pushy at all.