Many thanks for listening!
Unfortunately GarageBand has crushed all the life out of this piece with its very poor automatic mastering.
The subtleties and colours I created in the piano part have been obliterated!
It sounds much nicer when played back within GB itself, before the auto-normalisation has been applied.
Apple have also removed the ability to produce a score in the iPad version.
It won't even export a Midi file: so I can't make a score that way either!
I think the only way to do it, is with the full version of GB. So I guess I'll have to wait until I've bought a MacBook.
The piece probably ends too soon, as you said: so I think I'll try to expand it out a little.