Navigation disc 'high' for Mk IV system

[kvc]

Hi all,
Not sure, but I believe my version of the nav unit is a MkIV - it does say DVD on the front, but won't accept the DVD road map disc I purchased off eBay (professional). I see there are are a couple of sellers on eBay Australia offering 'high' discs (version 19), but for some strange reason they won't deliver or post - require local pick up only. They're all based in Sydney and I'm on the other side of the country...
If anyone knows how I can update my maps from the version 12 disc that's in my car, I would be in your debt. Do I really need to go to a dealer for this or can I simply get a burnt disc and use that? TIA