Soak the peas for 12 hours in cold water. Remove from the water and drain. Put the peas in a saucepan with the cold stock, bring to the boil, add the onion and simmer till the peas are very tender. Rub them through a sieve and return this purée to the stock, with the potato flour. Season with salt, pepper and the marjoram. Add slices of Frankfurt sausages, and the cooked gammon, simmer for a few minutes longer and, just before serving, stir in the sour cream.