Hi again. I solved my issue. I was examining the differences between a bot machine that the process worked ok on and the ther that it was failing with this error. I saw that the failing bot did not have these directories
C:\Users<YOUR USER>\AppData\Local\UiPath\Activities\UiPath.Excel.Activities.2.0.6382.14545
C:\Users<YOUR USER>\AppData\Local\UiPath\Activities\CsvHelper.2.13.5.0
Once I copied these across the process worked ok.
I concluded that the packages that your processes are dependant on at runtime must exist in this AppData directory structure. The primary difference between my servers were one had Studio installed (and therefore had installed all of the packages as part of the development process) and the other was a robot only and had never had any dependant packages installed. It is still not 100% clear to me as to the best process for deploying these to a robot machine as copy paste of files seems a bit low tech! But at least it works now!