Put the marrowfat peas and bicarbonate of soda in a large heatproof bowl and cover with boiling water. Leave to soak for 12 hours or overnight.

Drain the peas and rinse them twice with cold water to wash off the bicarbonate of soda. Put the beans in a saucepan and cover with 650ml cold water. Bring to the boil, reduce the heat and simmer for 30 mins until tender, stirring occasionally.

Mash briefly and stir in the butter, mint and lemon, seasoning to taste.