Thanks, finally found it.
For some reason, on my Mac, it compiled, meaning I didn't consider that the order of my libraries could be the issue (because, as I say, it was linking quite happily). This also reinforced by miss-remembered belief about what order the libraries (mine vs. sfml) should be listed in. Basically, I needed to put my library first because it depended on SFML, and I had it after (because I have a bad memory). I knew it would end up being something that trivial, such a pain that having the wrong order worked elsewhere.
hey ho.