Marsh tit or willow tit?

I've consulted several knowledgeable birders on Instagram and Threads, plus all the bird guides that I have, and I'm still not 100 per cent certain of the identity of this bird, although the balance of opinion strongly favours willow tit. The conclusive evidence would be its call, but twice when I've been back to record it with the Merlin app the bird has been there but silent. I'll have to wait for a month or two, until birds become vocal again.

It comes every day to bird food left by someone on a large boulder beside the Deerness Valley Walk, a former railway line that runs through the Deerness valley in County Durham.

There's more about the status of willow tits in the county at this Durham Bird Club website.