More annoyingly it doesn't reference the guilty package in the onscreen error. After checking the many log files CCM generates I found within the RAM drive path: x:\windows\temp\SMSTSLog resides the relevent SMSTS.LOG file.
The full error I recieved was after selecting the relevant OSD TS; resolving polices and then BAM :
I also noticed (and please use SMS Trace to view error logs):
I found it was a Lenovo Graphics Driver Package causing the issue; it appears SCCM has yet another little "quirk" where the hash isn't written sometimes upon creation of a package and the initial ditribution. If the version number of the guilty package is "1" try right clicking on the DP of the package and select "Update Distribution Points".
After some time the Source Version will increment to "2" and the date will be current - give your OSD another try and then see if it works - if it fails - check the log file again as this time the guilty package may have changed and it may be a case of trawling through a few before you resolve the issue.
I hope this helps.
Many thanks to J over at ManagedUX for the pointer!