I have to pick up one or two one things here, although my program only runs on Windows

1. " So, once I've found a tune in abc format, I can convert it to MIDI (using the folkinfo abc converter), then use NoteWorthy to transpose or otherwise mess about with it."

Why use two programs when one (HARMONY) will do both? Import MIDI or abc, transpose (and play, edit, etc)

2. "BarFly has a feature that lets you animate playback of the ABC source ("Highlight Note Played" from the Play menu)"

HARMONY has a 'one finger' play option on the score, both forwards and backwards, giving pretty much the same effect. If you also use the menu option that shows the abc notes instead of lyrics, a similar effect is produced, i.e. you can see the abc note being played.

3. HARMONY does a lot of validation on the tune, whether imported from abc or MIDI, specifically including wrong length bars.

4. HARMONY has a drag and drop interface for putting notes on the staff.

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