Cosmos bipinnatus Sonata Series is dwarf, up to 30cm ..
https://www.rhs.org.uk/Plants/...
http://www.chilternseeds.co.uk...
That's the only one which fits out of the many listed on Chiltern Seeds, but it's no guarantee. The petals on yours do appear to be shorter and slightly fluted, with a larger centre as you noted Myriam.
http://www.chilternseeds.co.uk...
This site links to several sites with a list of the most used bedding plants at the bottom, I don't know what else it can be.
http://www.beddingplantsuk.co....
