I decided to change the thermostat as according to several apps using the ODBII the temperature never reached over 75ºC even driving 3000rpm for several miles. The standard thermostat should open at 82ºC and I suspect the car was on cold running all the time. Now it takes about 1-3 miles for the dash to read 90ºC, where previously it never got close. According to the ODBII reading it stays around 90-95ºC. Previously with a mixed month or work and the odd long drive down the motorway, I'd be lucky to get 450 miles to a tank. This equates to 38-41 MPG when calculated using brim to brim method. Although this month hasn't been a completely typical month, I've currenlty done 560 miles and the trip is saying I have 50 miles left and this is with a hot month and I've had the aircon on every time. Even if I dont get another 50 miles from whats left in the tank, Im still getting 100miles more per tank at least. All in all it seems a worth while endevor for me.