Hi Martin
I would be inclined to just keep an open mind about the id of the bird on your feeder. The latest thinking from Richard Broughton, the man widely considered to be the world's leading expert on the two species, is that visual criteria are essentially useless in the field. This is a tiny quote from p18 of his book;
"....especially in Great Britain too great an emphasis was placed on unreliable features in popular field guides, such as cap gloss, size of the throat patch and presence or absence of pale fridges on the secondary wing feathers."
He mentions features that can used for separation but these are so fine that they are of no use in the field only if you have one in your hand. In short, he considers vocalisations to be the only reliable method to identify them for most of us.
I live not far from you in Llangybi. I have recorded Marsh Tits a few times around Llangeitho. There are some quite extensive areas of mature deciduous woodland around the village which are preferred by that species. There is also a good population of Willow Tits not far away from you, kind of between Stag's Head and Llanio. They can be found in the boggy, overgrown ground up there.
I guess either could be using your feeder. Keep listening!
Ian
"The Marsh Tit and the Willow Tit" by Richard K Broughton 2025 published by T & AD Poyser