If this IS a Willow Tit, it's the first I've knowingly seen.
The habitat was birch woods.
No interactions present.
Unfortunately, the bib shape and the colour of the cap are not constant features. The first picture looks like a Willow Tit, with pale wing panel and no obvious pale cutting edge to bill. The second picture seems to show a pale patch at the base of the cutting edge, though, which would suggest Marsh.
Any other pics?
My Flickr photos...
Here you are Ian. They're all the same bird, as I recall - I've been wracking my brains to recall whether it could be MORE than one, but I don't think so.
The warden told me that they'd never had Marsh Tits recorded at the site, but that proves nothing, of course.
Assuming that they are all the same bird, then a couple of the pictures you've added make it clear that there is no pale spot at the base of the bill, confirming it as Willow. The white cheeks, large diffuse bib and pale wing panel back that up. The cap looks glossy, though in some pics, showing that that feature is not definitive.
The extra pics helped...the more the merrier, with species like this!
I'm quite chuffed. I've just spent a few days up in Lincolnshire, and not only was it super weather but I got to see my first Willow Tit!
Lat/Lng: 53.1333, -0.1513
OS grid ref: TF237611