'Slower and Lower' is the best motto.
Sommerlad chickens grow slowly and live for at least twice as long as supermarket chicken. Most chicken recipes are made for supermarket chicken, so generally speaking, you should lower the cooking temperature the recipe suggests, and lengthen the time.
A cooked Sommerlad chicken will always benefit from being rested for 10 to 15 minutes before serving.
A frozen Sommerlad chicken should be fully defrosted before cooking. The safest way to do this is in the fridge for at least 48 hours (we like a good 3 days). Defrosting in the oven can ruin the texture of the meat.