Anyone can advise where and how can I modify the current light switch (one gang clipsal) to the timer control light switch, which I can set to on & off the light.

http://www.farnell.com/datasheets/51984.pdf

If I use Hager. There will be a ugly timer beside the existing light switch. Nit sure there us anything that can replace the light switch... With simple timer switch just like a normal time switch and flat enough... Ti replace the standard clipsal switch.