If you want the job done for you try Ouse Valley Autos in Warboys near Huntingdon.
http://www.fowl.org.uk/directorydetails/directorypages/Sponsors/ouse%20valley%20090221.htm
They are independent Fiat Specialists and Fiat main dealer trained.
They have done the job lots of times and are advocates of keeping the box slightly overfilled to ensure the gears get adequate lubrication.