Disk Space in Library/Containers directory

I am short of space on my MacBook Air. I have managed to track this down to a 61GB Library/Containers directory

MacBook-Air:Containers roychapman$ pwd

/Users/roychapman/Library/Containers

MacBook-Air:Containers roychapman$ du -sh .

du: ./com.apple.mail/Data/DataVaults: Operation not permitted

 61G .

I have gone through thsi directory and nothing is that large. the three largest (visible) files are these

1.6M com.apple.iWork.Keynote

1.3M com.apple.photomoments

1.2M com.apple.CalendarAgent

But of course this error suggests one directory is not visible

du: ./com.apple.mail/Data/DataVaults: Operation not permitted

Not sure how I can make this visible (yes, I am root). There are no hidden files that I can see (at least at the base of the directory).

I have tried Apple support and get the sensible, but not very helpful, "don't touch this directory". But 61G is huge, especially as it is unaccounted for. Any pointers. Anyone?

I'm running Mojave, but also an issue on High Sierra.

Roy