Took a little doing to get WB uninstalled since I couldn't switch to the default Windows UI, but managed to get there. Had to stop the WB service and end the WBCore process even though WB had been "unloaded". Initial uninstall aborted due to lua5.1.dll being missing but copied it to the WB folder from another SD folder & the uninstall proceeded to completion. Used Revo to clean up remainders.
Downloaded a new copy of WB 8.12 after a reboot and it installed successfully. I can now apply skins again. And my cart is empty!
Not sure how it got corrupted but that appears to have been the case.