This shows the benefit of catching a cold! I'd almost given up on 4267 (only a few clues left to solve but unsure of some misprints, something amiss with the 5 clues though I had two titles, one title seemed to be of a film not a piece of music...)
Went to work Monday but left early feeling rough. Looked at 4268 on bus home, spotted feature of Group I clues, made good progress.
Knowing I'd be staying home Tuesday, I found the patience to return to 4267 Monday evening and completed the thematic description. This explained a lot, including what was going on on row two. Surprised myself by actually finishing the puzzle ready to post the next morning. Only then saw the other half of what to do to the five clues.
Tuesday, plenty of time, breezed through 4268 until I reached the final instruction. Baffled. Had a look at this site and realised it might be worth looking for the relevant quotation. This pulled everything together and I was able to admire the construction - an easier puzzle than 4267 but elegant and satisfying.
Posted it today, did the weekend's Azed, but I'll have to go back to work tomorrow as I've run out of crosswords!