The bird in question was wearing a yellow darvic ring, number 089.
Mute Swan 089
Whilst there, he also recorded a pair of Gadwall (his first record at this site), 2 Kingfishers, 2 Pied Wagtails, a single Lesser Black-backed Gull, Black-headed Gulls, and, best of all, a Water Rail!!!!
Update on 17th Feb: Luke and I've just been down to Holmebrook Valley CP again, and the swan is still there, with the female bird.