In a saucepan, cover the yellow peas with the stock, bring to the boil then simmer for 30 minutes (check now and then, to make sure it does not run out of stock and burn and top if necessary).
Dice the chicken, onion, garlic, carrot and celery.
Splash olive oil into a frying pan and bring up to the heat and toss in the chicken, vegetables, sage and the yellow peas (drained of excess fluid) and then season with salt and pepper and cook for 5 to 8 minutes and then allow to cool.
Line loaf tin with pastry, overlapping slightly and then fill with the chicken mix and then fold pastry in and brush with milk.
Score the second sheet of pastry in a diamond shape and place on top as the lid.