The number 1 is the trip function of the computer, all details in this function will reset to zero when the ignition has been turned off for 2 hours or more. If you press the reset button on the lower edge of the wiper stalk the display will change and show a red 2 which is the accumulative function of the computer and as it's name implies it will keep a running total of miles travelled average speed, average mpg until such time as you want to reset the computer yourself.
By pressing the reset button briefly the display will change: Trip 1, Accumulative 2 , Environmental, Nav display (if fitted) and digital speed readout.
You will also see the 1 or 2 below the speed if you have the digital speed setting on the DIS and by using the rocker switch on the end of the wiper stalk you can scroll through the computer readouts.