There are two holy grails of air travel, moments of such unexpected serendipity that they are recounted to friends and family for years to come. The first is the free upgrade, when, usually at the check-in desk, you’re informed that, due to your frequent-flyer status – or simply your dashing good looks and Savile Row suit – you needn’t spend the next nine hours with your kneecaps in your face like the rest of the plebs.