C209 - satnav upgrade to get 7 digit postcode..

Guys i have read the other posts on this in relation to other Merc models but i claim ignorance in trying to work out what version of Comand i have in my car to then work put if i can just get a firmware update or not from my local dealer which would enable a 7 digit postcode look up as opposed to the default 5.

How do i know what version i have?
Is their a minimum version that is upgradeable to use the 7 digit codes?
Is this something a dealer should do relatively cheaply or should i expect a sizeable bill?

Thanks in advance...