Just received this from OL Care:

You will need PReS v6.3.0.2 to be able to generate the new Royal Mail Mailmark barcode with the correct C40 encoding. A new barcode type named CMDM RoyalMail DataMatrix has been added to the PReS Barcode Wizard. Please use this module instead of the standard Datamatrix module.

PReS v6.3.0.2 can be downloaded from the following FTP site
Host: ftp.emea.objectiflune.com
User: <Contact OL care for user name>
Password: <Contact OL care for password>

Folder: ftp://ftp.emea.objectiflune.com/PReS/v6/

Hope this helps.

Edited by Marcel Küng (04/27/15 08:41 PM)
Edit Reason: Post contained internal FTP user name and password. Removed at request of Jeff Klein