I think I have it now, which has helped me get 13ac; a definition I didn't know. I've eaten now, so can keep at it a bit longer, but I'm not enjoying it.
Sarah,
The "theme" is a well known Scottish group from the sixties. I am happy letting you know this since it is of absolute no help with regard to any of your other questions, sorry.
I've a "supergroup" from the 60's whose name indicates "the best". Apparently the clue is also the key to the theme, but much of this remains obscure to me.