Cheers for all the info guys - calculated it at 30.9mpg after 200 miles mixed motorway and town driving :-(

The thermostat was replaced very recently as it was leaking - I don't have any issues with the temperature gauge... Sits nicely in the middle. Probably takes around 5mins to get there. Although I am intrigued by "both" thermostats... Two?!

I've also cleaned the MAF sensor (the one near the air filter?) when I changed the hose as it was filthy - sadly made no difference.

Next ill be checking the brakes aren't binding but none of the wheels seem particularly in comparison to the others. Still... I'm desperate to solve this as its so frustrating!

I've downloaded that app too - seems very useful so will definitely make use of it, cheers!

Some great info coming out of this, cheers fellas