Heat oven to 220C/fan 200C/gas 7. Cut the potatoes into 2 mouthful-sized chunks and put in a large pan of salted water. Bring to the boil, cook for 5 mins, then drain. Meanwhile, place the olive oil in a really large, sturdy roasting tin and heat for 5 mins. Tip the potatoes in and cook for 20 mins.
Meanwhile, peel the zest from the lemon (a vegetable peeler is great for this) and slice it finely. Mix it with the sea salt. Place the chicken on a board with the legs pointing towards you, and undo any string tied around. Lift up the skin around the cavity and work your fingers under the skin over the breast and legs, easing it away from the flesh. Push the salty shreds of lemon rind under the skin, spreading it as evenly as possible, followed by four sprigs of rosemary, one on each leg and one on each breast. Then put one sprig inside the bird along with the peeled lemon.
Take the tin out of the oven, give the potatoes a good shake, and then arrange them so you can fit a rack over the roasting tin. Then place the chicken upside down on the rack and roast for 20 mins. Turn the chicken over and turn potatoes, coating them in the fat that's run off the bird. Return to the oven and roast for another 20 mins, then turn the potatoes again and cook for another 20-30 mins, until the chicken juices run clear.
Remove the roasting tin from the oven and put the chicken on a serving platter, covering it with foil to keep it warm. Leave to rest for 20 mins. Meanwhile, scoop the potatoes into an ovenproof dish and return to the oven to finish crisping up. Pour away the fat from the roasting tin, then put the tin on the hob over a low to medium heat. Add half the stock, then bring to simmering point, stirring and scraping up the brown residue. Boil, reducing until darker and syrupy. Add the rest of the stock, and bring to simmering point again, bubbling to reduce it. Taste and adjust seasoning. Pour through a sieve into a warmed gravy jug.
Pile the potatoes around the chicken, and garnish with remaining rosemary sprigs. Serve with gravy and braised lettuce.
Freezing: Freeze the chicken at the end of step 2, for up to a month. Defrost thoroughly before continuing with the recipe.