I thought it was about time to make an accurate measurement.
From the best part of a full tank (brim to brim), my stats:
Fuel Used: 32.12L
Mileage: 343.5m
Real MPG = 48.6MPG (UK Imp Gal)
Car gave me 50.4MPG so over estimating by 1.8MPG which is not too bad.
Car is a 2006 1.4 with 80K, recently serviced including all 8 spark plugs.
Driving was very mixed over this tank (short, long, urban, motorways etc)
Fuel is Sainsbury's Supermarket 95RON
EDIT: Reading above it seems I'm getting a little less than others, and my car reading is a bit further off than others. I don't think either is enough to be concerned about.