MAXTRANSFERSIZE setting causes restore to fail
This turned out not to be an error with the backup, but with the restore command that I was using.

RESTORE verifyonly  FROM 
DISK = @backupFile
 WITH FILE=1,  STATS=5,  MAXTRANSFERSIZE=4194304


returned the error "Damage to the backup set was detected"
some
whereas

RESTORE verifyonly  FROM 
DISK = @backupFile
 WITH FILE=1,  STATS=5


returned "The backup set on file 1 is valid."

Which is ironic given that I was only using the MAXTRANSFERSIZE option to try to speed up the restore, but instead it cost me half a day!

The error only occurs on backups taken from last night onwards.  Earlier backups verify fine whether I specify MAXTRANSFERSIZE or not, which is strange, but I guess I can live with not knowing the reason why that is unless somebody has an insight into that behaviour?