To close this and in case anyone else has the same problem, it appears that a scheduled backup (call?) in NTBackup MAY have a limit on filename length.
I was trying to save just the 'Outlook Express' folder in 'identities' in 'docs and settings'. It worked on a single 'one off' backup. It did not on a scheduled backup. I am now successfully backingup the actual master folder {xxyy12346 etc} daily in scheduled. I can only assume that the schedule command had exceeded its allowable path length? Maybe there is a 'legacy' 128 hangover in 'Scheduled Tasks' from DOS days for a bat file ? Version I have is schtasks.exe 5.1.2600.5512 in Sys32 folder.