Hi Graham,
not quite sure I understand.

To have the single touch settings change with the variations, all you need to do is press the the Style Touch Button & make sure it keeps flashing
Or are you then coming up with incorrect melody sounds for your song?

If so, choose sounds for each of the sts's & save
ie

just say you've got your style, sts1, vari1 you change from organ to piano etc etc
you have to write ( save) sts1. Then you do vari 2 sts 2 change your sounds & save to sts 2. etc

If you could elaborate a bit more, as I said I'm not quite sure of what the problem is.

best wishes
Rikki