How do you collect valuable art in London? In the same way most of us manage to live in this city – downsize. At the Art on a Postcard auction, there’s a chance to bid on itty-bitty works by famous contemporary artists at a starting price of £50, but as with most things in life, there’s a catch. You won’t know who has made what until after the bidding has closed. You could be placing the contents of your tax rebate against an original Maggi Hambling, or the work of an unknown Central Saint Martins student. Either way, you’re doing a good thing – all proceeds will go to the Hepatitis C Trust.