So we were forewarned that this might be a problem. With this in mind, it has been our intent to work him up to trailering slowly. We did some training with him involving leading him up to the concrete pad that is the horse washing station, which is apparently just as terrifying to Dubbin as horse-floats are. After a couple of sessions of persuasion we got him to the point that he'd stand on the concrete pad, and on one memorable occasion he let me hose him down as he stood on the concrete while M held his lead-rope.
Today we attempted the next stage, involving an actual horse trailer. The trailer itself belongs to the person mentioned in the first paragraph, who has horses agisted at the riding school; and she was very helpful in offering lots of advice on how we should go about this. Dubbin certainly wasn't terribly happy about going near the trailer initially, and did his usual stroppy display of "you can't make me!" But with M holding his rope, both of us making soothing noises, and a lot of persistence, eventually Dubbs settled down a bit, and we started working on getting him to walk onto the trailer, not just near it. We didn't expect to really manage it in one session, but the thinking was that if we could at least get him comfortable standing on the trailer ramp, or get him to be okay with sticking his head into the trailer itself, we'd count that as a win.
Imagine our amazement when, after half an hour of persuasion, a bucket of oats, a kilo or so of carrots and a couple of apples, Dubbin walked voluntarily right into the trailer! Yep, all the way in! Admittedly he didn't stay in there for very long... just long enough to snarf another carrot and a mouthful of oats. But this "untrailerable" horse had walked onto the trailer, under his own steam, and without any panic or stress. You could have knocked all three of us down with a feather.
He's not yet quite at the point where we could lead him onto the trailer and expect him to just stand there indefinitely and be completely relaxed. (We'll save that for the second trailering lesson!) But that's one HELL of an achievement, for him and for us. I confess that I was humming "Look at my horse, my horse is amazing" (https://www.youtube.com/watch?v=hyIiixA0ez0 ) all the way home.