An update to share how I got on in case any other PPRuNer needs to tread the same path.
First of all I ran the script found
here to install both Optware and a bunch of useful features on my WD "White Lights" NAS. Included in these features wasn't just Cron itself but also a nice folder structure to allow any required executable job to be dropped in the appropriate one for the frequency required (/etc/cron.min, /etc/cron.5mins, /etc/cron.15mins, /etc/cron.30mins, /etc/cron.hourly, and so on)
I consequently have my temperature and load logging script sitting in /etc/cron.hourly and producing a new log entry every four hours thanks to a time-checking "IF" statement that I placed in the script.
Another feature included in the download was mini_sendmail so I have written another script that runs every 15 minutes and which checks if either of the two HDDS in the NAS are approaching their maximum design temperature of 55 deg C. If they are, the script connects the NAS to the mail.btinternet.com server to send me an e-mail (which I will receive wherever I am on my Blackberry).
If either of the HDD temperatures continues to climb and exceeds 55 deg C I will be sent a second mail, and if no cooling action is then taken within five minutes that results in the temperature dropping back to 55 deg C or below, the NAS will shut down.
On a scale of zero to hero I'm only just off the starting blocks but considering that a week or so ago I knew nothing about Linux other than that the NAS used a stripped down BusyBox flavour of it, I'm pleased with these practical tweaks.
BTW - I highly recommend
WinSCP for connecting to a remote Linux device. It is an open source free SFTP and FTP client for Windows that also supports the legacy SCP protocol and that allows safe and simple copying of files between a local and a remote computer. It saved me MASSES of time compared to navigating around in an SSH command console.