The behind-the-scenes content shared from Lily Allen and her team for ‘West End Girl’ gave the public a rare insight into the usually unsung work of songwriters. What is it like to be involved in such an immensely emotional and personal project, Hannah Ewens asks Lily Allen co-writer Chloe Angelides, and Jessie Ware and Perrie Edwards co-writer and singer-songwriter Nina Nesbitt