What looked like 1 male gadwall with the usual group of mallards, being fed bread by a man on the canal path by railway bridge near Fairfield and college farm. The man said he had seen it a few times. I didn’t get a long enough look to see whether there was a female gadwall or it had latched onto a F mallard.
I’ve also recorded a solitary Gadwall drake where you describe and also half a mile further South, where the footpath from Ashton Road joins the towpath, during WeBS core counts this Winter – presumably the same individual.
The man feeding the ducks said he regularly fed them at that spot and that the drake gadwall would always be following a female mallard. He’s not a birder but who knows…? Perhaps imprinted at birth or a bit of a personality?