Hi Mike, i might be a bit late on this one as i was away. Anyway, we had this task just three weeks ago. dependant on which gearbox you have(722.6 or 722.9 7 speed) i found what you need.
Google "722.9 7g-tronic", the first file is an
americal who has written his own pdf file at bottom of page plus we has also put offical merc.
service litrature there as well which you can print off and file.
If it is 722.6 5 speed go this link,
www.mbclub.co.uk/forums/engine/88418-how-change-
automatic-gerbox-oil-flush-722-6-box.html
He is a merc specialist on outskirts of Portsmouth and the photos are crystal clear.
the going rate seems to be anything from £150-£225.

Regards
(p.s. i have not made spelling/grammer mistakes in link address)