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Tip the barley and oats into a bowl, pour over 1 litre boiling water and stir well. Cover and leave to soak overnight.
The next morning, tip the mixture into a pan and stir in the plums. Simmer for 15 mins, stirring frequently and adding a little water if necessary to get a consistency you like.
Stir the vanilla into the yogurt and serve on top of the porridge with the seeds sprinkled over.