[Quacker]

The increase in traffic and probable slower progress surely had more to do with it than air temperature.
My Honda consistently does between 42 and 44 mpg on a long journey, Summer and Winter. Except once when I travelled from near Felixtow up the M6 and over to Shrewsbury and across to Aberystwyth in very heavy holiday traffic. This kept speed on the motorways down to no more than 50mph for long stretches and seldom over 60, more often down to 30 and 40. Was quite tiring but it did yield 50mpg, a figure I've not got within 5mpg of in three years since.