For the Frenchman Marcel Proust, the elixir of memory might have been a petite madeleine, but that wouldn’t work on British-bred me. What I needed was a can of Heinz cream of mushroom soup and a packet of Sainsbury’s cheese and onion crisps. As I gathered these and other long-neglected childhood foodstuffs from the supermarket shelves, I thought surely one sniff, one taste would be enough to take me right back. But to what, exactly? And how?