Not seedy stuff, but I couldn't see anywhere more suitable and thought the information might be of interest to others.
I learned that it is possible to propagate Cardamine pratensis from leaf cuttings (like Begonias and Streptocarpus). The seeds are described as small and difficult to collect, so this is the recommended method. You just collect leaves, lay them on damp compost ensuring good contact, cover to keep the atmosphere humid and wait 2-3 weeks.
I was a bit sceptical, but I've got some tiny shoots emerging, so apparently it does. No roots visible, unlike in the photos I've seen posted online, but I presume they're there and don't want to disturb things to investigate.
This website has several photos, it suggests using basal leaves, but that doesn't seem to be essential - I've got shoots from the narrower leaves further up the stem.
Another website also has a photo showing success with stem leaves and gives a bit more background on 'leaf vivipary', including the information that floating the leaves on water should also work.
I've no idea whether this method works with other Cardamine species.