The most reliable difference between de Marsh Tit (Poecile palustris; in Dutch: Glanskop) and the Willow Tit (Poecile montana; in Dutch: Matkop) is the white patch at the base of the upper mandible of the Marsh Tit (clearly shown on the photo).
Willow Tits have an all black bill with no pale bill spot.
However, this white spot is not always clearly visible, but it is in the photos above, especially the first one.
Note the cheek pattern on the bird: whitish ear-coverts contrasting with a cold grey-brown wash on the neck sides and rear of the cheek patch is strongly indicative of a Marsh Tit.
In other words: many Marsh Tits appear to have a much smaller white ‘face’ than Willow Tits.

1st photo: Bird banding site, Naarden, The Netherlands, 8 January 2006
2nd & 3rd photo: Zanderij Crailo, Hilversum, The Netherlands, 30 October 2023
4th & 5th photo: In my yard, Hilversum, The Netherlands, 23 January 2022