Songwriting and deadlines shouldn’t ordinarily mix; no great records are made when an artist lets the idea take its natural course. But when a deadline comes in the form of a seminal summer gig, then maybe exceptions can be made. For as long as rock and roll has been around, the summer gigs haven’t come much bigger than Knebworth. It’s quite simply hallowed turf within the genre, playing host to some of the biggest and most impactful gigs of the…