Actually it's because seahorse was storing the key in it's cache. Reboot and it rejects my login. \o/

Do you ever just have one of those days? I've been 100% over thinking this and trying to make it more complicated than it needs to be. Anyway, problem solved, it's now happily rejecting my attempted to log in! If I replace the key with the backup, it works.

Thank you so much for being so patient with me. Very kind of you to persist with the help. My battery is nearly dead on my N900, but as far as I can think, everything's all sorted now.

Thank you once again.