Soak the chickpeas in cold water overnight. Drain, put in a heavy saucepan and cover with water. Add the baking powder, bring to the boil and cook for 10 minutes. Reduce to a simmer, and cook for 1 1⁄2–2 hours until the chickpeas are soft and mashable.
Drain the chickpeas, put in a bowl and mash with a fork. Add the remaining ingredients and 200ml/ 7fl oz water. Mix well and chill for at least 3 hours before serving.