Thanks to all for suggestions & comments.
I have frozen this design now, I will be using following configuration.

The AC coming from a 6-0-6 center tap transformer is rectified using two diodes. This rectified pulsating output is smothened to true DC by a 100µF. This true DC output is then scaled with a multi turn pot. This scaled output is then clamped to Vcc & GND with two clamping zener diodes to ensure that it is in 0-5V limits. Now this is fed to ADC to measure the voltage.

A special thanks to Erik for pointing out the opto rendundancy.

Steve/Craig please tag this thread with a smiling face and close it.

Regards,
Prahlad Purohit