Heat half the oil in a paella pan or large deep frying pan. Add the chicken thighs and chorizo and brown, turning. Stir in the garlic, onion, pepper and cook till softened. Stir in the rice.
Add the chilli flakes and reserved mussel cooking liquid, chicken stock and soaked saffron and liquid. Stir well, bring to boil and simmer gently for 10 mins.