Hi again,
Just checked the video settings and they were set at 1024x768 with a refresh rate of 60Hz and the flickering is still continuing approximately every 12 seconds.
I've checked the Dell website to see if there is an updated video driver for Linux but whilst there is an updated driver for Windows there is nothing for Linux so I'm not able to update the video driver unless there is something within both CentOS and Ubuntu that will allow me to cure this flickering.
Graham