The error as it was displayed on the clients screen was:
This task sequence cannot be run because a package referenced by the task sequence could not be found. For more information, please contact your system administrator or helpdesk operator.
This happened immediately after selecting the task sequence and processing the dependencies.
Upon viewing the SMSTS.log we spotted the error
Failed to resolve selected task sequence dependencies. Code(0x80040104)
The only change made to the TS since yesterday was a new driver package for the Lenovo X220T tablets. The reference to a missing package reminded me of a post I made once in reference to drivers and driver packages - that if you leave the distribution point on Source version 1 it will just bomb out. Refreshing the DP didn't help so went via "Manage" and created a new source version.
As if by magic..... It worked!