My normal breadmaker recipe uses milk as the main liquid ingredient. Normally I use fresh semi-skimmed milk for this and get great results. However, at the moment I only have "long-life" (UHT treated) milk. Is this an acceptable substitute on its own, and if not, is there a way I can tweak the recipe to work better with this milk?
Yes, you can definitely use UHT milk in bread.
UHT is not usable for some applications, because its proteins have been changed by the heat. But in bread, you don't need the proteins from the milk. Bread recipes include milk to make the dough richer, so it is mostly the fat content that matters. Using UHT won't change anything in the dough structure. I even think that the usual unpleasant taste won't be noticeable, but I am not so sure about that. Still, it gets diluted a lot in dough, so if you can drink UHT as-is, you shouldn't have a problem with the bread taste.