I thought the setter was being overgenerous in giving us the word breakdown of the perimeter message and 25 corner letters.
I would have preferred more of a challenge.
Thanks for the explanation of canon. My copy of Chambers (the latest edition) does not state 'a piece of music similar to a round' but the on-line word wizard dictionary does. How bizarre!
20A I went for "MISTER" rather than "MASTER" because of the quotation "How pleasant to know Mr. Lear". Is it a toss-up, or will either be accepted? Not clear about 27A: Songbird in Snowdonia possibly left for Portugal. L_R_. LARK seems obvious, but don't see why.