Banksy in Cambusbarron?

There are the remains of a few sheds and other buildings in the old Murrayshall Quarry, the focus of the current campaign to stop re-activation of quarrying. Walking through today we came across this rather fine and startling piece of "street art" on one of these. How and when did it get here? Do we have a secret supporter?
There is some real artistic skill here , is there not?