Thanks.
Since /var isn't easily accessed using file sharing I thought it would be useful to have a scheduled task to copy the latest /var/log/messages to somewhere that is. The task can be disabled and run as and when required. As 'root' user the script I added directly to the task's script box is
Bash:
#!/bin/bash
cp /var/log/messages /volume1/<shared folder/subfolder>/messages.log
chown <adminuser>:users /volume1/<shared folder/subfolder>/messages.log
I added '.log' since it's recognised as a text file but '.txt' could be used.
Using BBEdit on Mac I can SFTP directly to this file.
** That we can create tasks that run as 'root' without having to authorise them makes me uneasy. On the SSH command line an admin user still has to enter their password when using 'sudo' and these tasks should be the same. Some NAS will be managed by multiple admin users and there should be some log (e.g. out to external syslog server) that records when changes to tasks are made, by whom, and that they authorised it. It would be better to have a roles based admin setup so that some admins have limited powers and they can be permitted to do some activities when authorised by a suitably empowered admin user.