Averaging 53mpg on a trip from one end of Britain to the other might seem a creditable result for a diesel, if not a wildly impressive one. But averaging that in a Jaguar XJ is rather more remarkable. To prove the frugality of its largest saloon, Jaguar sent an XJ TDVi from Land’s End to John O’Groats – 840 miles – knowing that it would do well. But on reaching the furthermost tip of Scotland the Jaguar still had plenty in the 85-litre tank and so headed south, running out of juice at 1000.2 miles. The car was standard in every respect and was no poverty-spec special: it had satnav, rear DVD screens and a heavy-duty sound system.