Using a genuine 918 serial and mac I found, allowed me to generate a new pair which did work (by altering the grub at boottime using c to access the command line). However, I still have the same problem. No access to the ESXi interface as long as the DSM VM is running. If i launch the grub command line and type 'mac1' it shows me the one i updated to. (and sn also) However, if i go into system settings in the DSM UI, it shows me the old one. the SN is updated in the DSM UI.
I think I'll start again from scratch. I really need access to the ESXi interface while DSM is running.