It wasn't backed up - will have to follow advice advice I've given others ;/
I've not done file recovery since early days of WinXP.
On the affected machine I'm running Stretch(9.1) with Mate desktop.
The affected folders are on a partition normally mounted by a line in fstab.
I immediately shut down.
There is another instance of Debian on a separate partition.
Logging in as root I edited fstab commenting out the appropriate line.
I feel the the data is still valid - there is just no appropriate directory entries for the affected folder and its sub-directories.