Yellow Billed Magpie..? Surely not?

Hi all, On my commute yesterday morning I was sat at a set of traffic lights in Dunblane ( Central Scotland), I glanced to my right and was watching the Magpies in the tree about 15-20ft away. One had a very distinctive white/creamy bill.. but looked just like the other magpies in every other way. I got home and googled for a bit and the closest I can find is a Yellow Billed Magpie, but these reside in the USA. Maybe somebody could shed some light on this for me? I went back today armed with my camera, but obviously, there were no Magpies in sight. Andy :)