Banksy prints are almost always sold at auction these days with a Certificate of Authenticity from Pictures on Walls, the gallery which sells Banksy's limited edition prints. If you don't have one, Pictures on Walls will charge you fifty quid to print one out for you - or rather that's what they told me, and then charged my card £62.50. When I queried this they never answered my email. You might think that as they sell the prints they might include a certificate as part of the service, but this is a gallery who would charge you for wear to the carpet if they could get away with it.

This is of course if you manage to get them to reply to an email. It took me nine months of emails before they actually bothered to do a certificate for me, and that was only after I persuaded someone at Christies Prints department to contact POW on my behalf.

The lesson being - don't buy a Banksy without a certificate because Pictures on Walls won't get off their bottoms to do one unless you badger them incessantly. They seem to believe that because they are 'alternative' and gritty and urban, basic things like efficiency, customer service, and courtesy don't apply to them.