Dear Anne,
This advice is from the book 'The Genus Sorbus' by Hugh McAllister:
1. Seed of S.poteriifolia should not be allowed to dry, as in this species dry seed rapidly loses viability.
2. Seed will germinate at 1C. During the cold treatment the temperature should not be allowed to rise significantly for any length of time, and seed should be maintained at 1C until germination is complete.
He also mentions seed should be cleaned asap after harvesting of fruits, and the use of a fridge for germination, to ensure an even temperature.
He talks of putting the seed damp in petri dishes so you can keep an eye on progress.
It's a lovely book if you like Sorbus, but if you would like a photocopy of the germination pages, please let me know.
Giles