FWIW, my two cents worth.
Firstly this a Greek bird, so any idiosyncracies of British Marsh/Willow id might not necessarily apply.
It is a Marsh Tit, not only from the proximal spot on the upper mandible, but also because of a combination of other features, including straight narrow bib.(no flaring). Continental birders tend to see more variety in the range of features on a regular basis. As several people have indicated with their knowlegdeable input, local habits, re altitude also plays a part.
Since the proximal spot on British birds became the only single reliable id feature, people disregard other features. This is false logic, because if a combination of other features indicate one or the other, it is a good indication that it is indeed that sp.
Everybody agrees that it is a Marsh Tit, but not everyone explains why. I see too often the proximal spot argument being used, when the bird is cleary a Willow and has a bit of god knows what stuck on it's bill, or photo artefact falsely points down that route. All the birding experience of yesteryear is not wrong, those experienced birder's always used a combination of features to come to a conclusion. Todays beginners are advised to take this on board, I did and find it sound, sometimes the bill is not imediately obvious because of shadow etc. It is not a dig at anyone personally, just a general observation into the pitfalls of ignoring obvious features.