OK - I have solved my problem.  Following the two posts quoted earlier I changed my 
fstab entry to add 
uid=1000 and 
gid=1000.   The
 fstab is now -
# Network USB Drive
//192.168.0.1/USB                           /home/user/USB     smbfs   umask=000,uid=1000,gid=1000,guest,_netdev     0       0
I case it helps others the explanation of the entry is
//192.168.0.1 - this is the address of the modem
/USB - the name of the USB drive attached to the modem as the modem sees it
/home/user/USB - the mount point in the home directory
smbfs - the file system since this is a network drive using smb
umask=000 - sets permissions for anyone to write to files
uid=1000,gid=1000 - user and group ids
guest - I don't know what this is for but it is necessary
_netdev              The filesystem resides on a device that requires network  access
              (used  to  prevent  the  system  from  attempting to mount these
              filesystems until the network has been enabled on the system). See
 man mountHope all this helps someone somewhere 

This may not be a solution for others with different setups.
Alan