Been playing with this as its the first time I've had one. Has anyone done any long term observation of how accurate they are? I just did 2 long drives, london ec. On my Punto 1.3 diesel the trip computer told me I was getting about 56 to the gallon. When I filled up, my calculation shows above 60 slightly. Is this a big difference?

no,so many variables to PG from weight in the car to driving into the strong wind

Not only that you cannot calcute it correctly yourself, cause when you fill it up, you never fill it exactly the same, 2 deciliters off and there's a difference
Second, your trip computer calculates an average from when you've reset it, maybe you havent reset it, just looked at mileage and calculated the difference...and you were driving in a different mood in earlirer miles which consumpts more/less fuel...but I think you knew this second one