Michaelmas Daisies are a catch all name for the Asters which put on sprays of flowers roundabout now. Various species have been introduced to this country. In particular A. novi-belgii and A. novae-angliae from North America have become in effect wildflowers and spread all over the place, notably along railway lines.
 To all intents and purposes these two are indistinguishable from each other and equally prolific. I should note that the numerous American Aster species bar one have been reclassified and given other names which I should look up but I'm not going to. The Eurasian Asters for the most part remain Asters and the plant family for both continents is still Astereae.
 So Michaelmas Daisy works fine for me, though unscientific.