Fortnum & Mason's overnight shoulder-of-lamb shepherd's pie

Ingredients

• I shoulder of lamb on the bone
• 2 carrots, cut into 1cm dice
• 1 celery stalk, cut into 1cm dice
• 1 large onion, cut into 1cm dice
• 8 garlic cloves, finely chopped
• A sprig of rosemary, eaves finely chopped
• Leaves from 4 sprigs of thyme
• 2-3 tablespoons Worcestershire sauce, to taste
• 3 tablespoons tomato ketchup
• 500ml chicken stock
• a handful of frozen peas
• salt
• freshly ground black pepper
• 6 large baking potatoes (about 2kg)
• 200ml double cream
• 20g unsalted butter
• 6 egg yolks