Banksy valuation

Looking for expert valuations of how much the north London property might be worth now it has been blessed with a piece of work by Banksy. Land Registry documents show it was bought for £400,000 in 2012 and the owner estimates it to be worth around £1.3m today. Grateful for any suggestions of how the value might have rocketed with the addition of a much-sought-after piece of art daubed on the side.