Here's a very thorough article on the history of roundel use in the RCAF. Much of the article is also applicable to the RAF, since the two services used the same roundel until the maple leaf was introduced shortly after the Second World War ended.

There are a few places where the article contradicts what I have learned from previous research. Examples are the author's calling the modern RAF low-visibility roundel a "Type B", or where he misidentifies a marking on a Fairey Battle for the remnants of a roundel. Getting into the postwar RCAF and RCN, he transposes the maple leaf types a few times, mistaking one for the other in the text.

Minor errors like those aside, the article is very well done and the original colour photos are spectacular.

..indeed.. thanks for the heads-up!

yes, a jolly good read actually.