When I set up the crontab I accidently deleted the contents, but this was not really the issue.
I then set up my cron tab task, and thought it was not working.
I wanted to test my task by running it every minute, just to see it working, but I had set the times in-correctly.
I eventully learnt that to run a task every minute I had to use the below format
0-59 * * * * /tmp/my_script.sh
So it always worked, it was my lack of understanding...