There are so many variables to take into account: where in the country you are, where you have to keep the seedlings, what specific plants you are growing, etc.
How can you lie there and think of England When you don't even know who's in the team
I should clarify, when I've sown hardy perennials in September they've been outside from day 1, in a sheltered spot against the house or shed on the shelves from an old blowaway (the cover is long-since gone). Pricked out into small pots or modules as soon as they're big enough, then on to bigger pots in the spring when they're filling the small pot/module. Some die back in the winter then regrow in spring the same as grown-up perennials (don't panic!).
Doncaster, South Yorkshire. Soil type: sandy, well-drained
I don't have much luck with direct sowing in Autumn. A few things come up like weeds, such as nigella, but most disappear into the mud and the slugs. I sow HA's in trays in Spring and have much more success.
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When you don't even know who's in the team
S.Yorkshire/Derbyshire border
Also in South West so usually mild.
When you don't even know who's in the team
S.Yorkshire/Derbyshire border