Derek
Marsh/Willow must be one of the most frequent ID queries on BF, even with sharp, clear images
With the caveat that the image isn't really of high enough quality to be absolutely certain, I would suggest that Willow Tit cannot be ruled out;
A buffish suffusion towards the nape does not rule out Willow - distinctly two-toned cheeks would, but this bird doesn't show the obvious demarcation of a Marsh Tit (unless that's hidden behind the wire...).
Absence/presence of a pale cutting edge is not the feature to look for on the bill (despite many posts on BF misinterpreting the criteria posted by Poecile/Parus/KnockerNorton). Marsh Tit has a distinct pale
spot near the base of the upper mandible. Willow can show a pale cutting edge, it's a trick of the light, but Marsh has a pale spot that stands proud from the cutting edge. On the sharpest images I've seen posted on BF it's possible to identify Marsh Tits using that feature alone on the thumbnail images. On your image there is a suggestion of this spot, although the pale area on the bill seems to start at the base of the lower mandible and extends diagonally forward to the top of the upper mandible.
Other criteria are unreliable (and were shown to be so over a decade ago; unfortunately field guides, and birders, continue to promote these unreliable features).
(BTW Thornley Woods is not in Northumberland)
cheers
martin