Iona has offered up perfectly good advice and certainly knows as much and more as I do-(this may not be a compliment). As an additional source of info, though, have a look at the Jellito seed website where they also give you excellent germination advice...and for tricky seeds, they do a range called 'Gold Nugget' which have been treated to avoid the need for stratification/vernalisation. For many years, I have tried (and failed) to sow callirhoe involucrata seeds...until I ordered the Gold Nugget ones from Jellitoe (along with yellow flax (linum capitatum) and a few others. Amazingly, every seed has germinated. More than 100 of them! The seeds are not cheap but you get 10x as many as Chiltern, which, for all their good advice, are very stingy with quantities.
And yes, the (12) callirhoe seeds from Chiltern, which I ordered first, are still lying dormant in their pots. Sometimes, you just have to bite the bullet and pay for reliability. OTOH, I did order jasione perennis and perovskia 'Blue Steel' from Chilterns, which are doing well (if in minuscule quantities).
I used to order from Chilterns, many years ago, but fear they have gone off the boil a bit, preferring to send out glossy catalogues instead of renewing their seed stock. I swear, I have ordered from numerous seed outlets and now have an internal database of value and reliability. Some should be avoided at all costs, while some are a bit dodgy, but are also very cheap (Premier).I am entirely happy to share a list of reliable seed merchants...but sadly, one of my go-to vendors (Seedaholic) is now unable to sell in the UK, being Ireland based and I fear B&T World Seeds might be in the same position. My best source of S.African seeds (Silverhill) are no more after the owners were murdered by fundamentalist terrorists! Have also bought from Chileflora and Seedhunt (Californian wildflowers).
My absolute best advice, for anyone exploring the fabulous world of seeds is to collect your own. Provenance, viability and cost all come in well ahead of anything you can buy from any seed merchant anywhere, tbh. Plus, you get to spend the dreary late autumn and winter, sitting at a table cleaning next years promise. A lovely way to survive December and January.