I have included pictures of Marsh Tits showing a white patch on the upper mandible. It is noted by Svensson and reported by Dewolf in Belgium. He considers it reliable for distinguishing Willow Tit which lacks this feature. I have never seen any white on the upper mandible of Willow Tits. Could ringers confirm this please?

Further to your observations on the white mark on Marsh Tit upper mandible, I thought that you might like to see the attached photo of a Willow Tit which, as expected,  does not have the mark.