I only do my mpg calculations once the tank is empty and I refill. I think I am one of a few who fills the tank to the brim, but then I'm still getting over 400 miles to the tank. I do a lot of round town driving. I've got a 30 minute drive to work in which I'm probably doing 65mph for about 10 minutes, the rest of the time is town driving, 30mph and rarely into 6th. I try to drive carefully and get into 5th as soon as I can without the car feeling strained. I do drive like a grandad all the time to achieve my mpg and I know a lot of people say ' the car deserves to be driven ' and yes, but I'll drive it like this knowing I have the power if I wish to use it.
When I drove to Burnley and back, a 450 mile round trip, I did not stop to put any more petrol in and I averaged 32.9 mpg.
On Tuesday I put new tyres on which, according to blackcircles.com, claim I could savd upto 6 litres of fuel over a 600 mile journey. This would be a further increase in mpg of 2.2 on what I am currently getting, but I will keep you informed.
Check your tyre pressures/balancing/tracking/tyre wear. There may be nothing mechanically wrong with the car, but these can have an effect on mpg. But ultimately, boring calm driving, slow accelerating will help improve mpg.
Sorry for the boring reply. I like boring.