Photo @Annabelle Hickson
Most of today's chicken recipes have been developed for supermarket chickens (which are mere babes when processed at 35 days old). Sommerlad heritage chickens grow slowly and live for at least twice as long (closer to three for that matter) and have had time to develop "real chicken" texture and flavour. So generally speaking, you should lower the cooking temperature the recipe suggests, and lengthen the time.
A roasted Sommerlad chicken will always benefit from being rested for 10 to 15 minutes before carving and 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.
If you're looking for inspiration, here's a few places to start:
The Sommerlad family's original slow roast guide