SCCM Reboot during OSD Install - "NO MP Certificates"

I just spent ages troubleshooting an annoying thing with SCCM. Every time I tried an OSD PXE installation the PXE would load and then just before the task started it would just reboot.

I followed all the troubleshooting tips for Windows PE environment and reviews the "smsts.log" log to just see a message "NO MP Certificates" and a few similar floundering people on DL's

As a last straw before calling a proper SCCM expert, I checked all the Network Access Accounts in client agents and saw that the "Computer Client Agent" was not using the same account as the one used to install SCCM (although it was a domain admin!).

Switched this to be the same as the main install account and all worked fine ;-))