Update.
Friendly local MOT station gave me this procedure from the Autodata system:-
Switch ingintion ON.
PULL and hold trip button until the message "Service" appears in the display.
Release button.
WITHiN 5 seconds, PULL and hold trip button until the message "Service in......miles......days" appears.
Release button and switch off ignition.
This worked for me in so far as it has extinguished the spanner. Actually, I did not see "Service" or "Service in......", but I did see a figure of 9000 in the last operation. I'm assuming that the spanner will come on again in 9000 miles, but could be wrong. If it's right, then that will be fine since an average annual mileage for it will be around 7K when it will be serviced again, and the reset done for a further 9000 - I hope!
Thought this may be of help to someone at some time.
It appears that if the car is to continue to be serviced on a Longlife schedule then it has to be plugged in to the diagnostics to set up the next correct mileage/time service interval.
Kind regards,
Gareth.