Thanks for pointing me in the right direction. I can confirm that the process is as follows:
1/ With the ignition turned off, push and hold the trip computer button at the end of the wiper control stalk.
2/ Turn the ignition key to position 2, whilst continuing to hold the trip computer button.
3/ The speedometer display will flash, first with mph and then kph showing above the "0", continue to hold the trip computer button until the flashing has stopped, then release it. The speed and mileage will now display in kilometers.
Repeat the process to change the units back to miles.