Thanks. IdePix did run on the resampled L1C product. I then opened Mask Manager, on the resultant product (resampled_idepix). However, none of the selected cloud masks did appear on the image, so I'm really questioning whether any masks were created at all, even considering there are obvious cloud areas and their shadows.
Interestingly, if one clicks the top row in the Mask Manager, labelled as 'Invalid pixels', all the scene is selected:
I do have the latest SNAP version installed with all the recent updates.