Many of Nigella's recipes, and guest recipes, ask for a 3.5-4lb chicken for roasting. I'm a good and creative cook, and while I've cooked with many different chicken parts and have also taken apart a whole chicken for soup from scratch, I have not roasted a chicken whole. The bird I was able to find is 6lb and I just don't know which adjustments to make in terms of cooking times (and perhaps temperature?). Thanks for your help!
Most chickens sold in supermarkets are in the 3-4lb/1.5-2kg range so this is the size used in recipes such as Tagliatelle With Chicken From The Italian Ghetto (from HOW TO EAT). The roasting time for a larger bird will be longer, though generally you don't change the oven temperature. Also remember that a chicken straight from the fridge will take longer to cook than one that has been out of the fridge for about an hour before cooking and is allowed to come closer to room temperature.
The rule of thumb for a roast chicken direct from the fridge at 200°C/180°C Fan/350°F is to allow for a cooking time of 20 minutes per pound or 45 minutes per kilo, plus an extra 20 minutes. If the recipe calls for a different oven temperature then calculate from the recipe how many minutes per pound or kilo the cooking time allows and apply that to your larger chicken to work out the longer cooking time. Always check that the chicken is fully cooked by testing the chicken at the thickest part of the thigh. When the chicken is cooked the juices from the chicken should run clear (with no traces of blood) when you insert a small, sharp-bladed knife into the thigh, or if you use a digital or instant-read thermometer then it should read at least 75°C/165°F.