EDIT: Found out how to do it now. Just kept pushing buttons
For those wondering why I needed to change the distance read further.
I have changed the diff to a lower ratio for more efficient cruising. This threw the speedo off, so I have re-calibrated it. However the odometer is tamper proof and works of a different circuit. Presumably this is to prevent people dodging the diesel tax (which here is charged in distance driven, not at the diesel pump) Consequently it reads 5% low.
Ironically, being unable to calibrate the odometer results in me paying
less tax than I should. Go figure.