Every Halloween, thousands of perfectly edible pumpkins are sacrificed to the carving gods, only to be dumped and forgotten once the trick-or-treaters leave. Chef Mike Davies thinks it’s a waste – and he’s right, says Hannah Twiggs. With 18,000 tonnes of the stuff rotting in the UK each year, his new cookbook, ‘Cooking for People’, makes a strong case for pumpkins as a winter staple