Upgrade Mk3 to v25 to ensure Mk3 is running at 3/1-63. Don't worry about language yet.
After it is running at 3/1-63, use NavCoder again to set languages. Make sure the two are different (eg IT and GB) and select Female voice only.
Nav will ask for OS disk, insert the same v25
New languages should be loaded.
Make sure you re-code the Telematics settings, they get reset with new language loading.