Just had a strange thing happen on a routine journey of about 10 miles on quiet rural roads. As usual the engine temperature gauge did not start to move off cold till about 4 or 5 miles but as it reached three quarters of the way to normal it then dropped back to the quarter way position before increasing again to normal after about 7 or 8 miles. I then noticed that the indicated MPG figure had dropped to about half it's normal value (35 instead of 60) for that road and also when slowing down for a junction still in gear but with foot off the gas pedal the indicated MPG was 50 ish when it would normally be at it's max value of 99. When the car came to rest the engine tick over speed was about 1050rpm not the normal 750/800rpm, also the stop/start didn't stop the engine. I continued to my destination 2 or 3 miles further on but the symptoms remained as described. I returned to the car after about 20 minutes and set off on the 10 mile return journey home to find things behaved perfectly normally with the engine tick over speed and MPG reading correct. The only fault remaining is the engine stop/start still fails to stop the engine at all the junctions where it previously did. All the various settings and temperatures are as they have been for the last 1100 miles and everything has worked flawlessly. Any ideas or suggestions welcome.