Resetting the digital 'service due' indicator

Don't all jump in at once, but I've forgot how to reset the digital 'service due' warning on my e300 '97 diesle estate my mechanic who serviced the car has gone on holl's and didn't reset it. He did the same thing last time. He told me on the phone how to do it then, but senility has set in since then.
Cheers to one & all in liew