Hi Saugata,
sorry I hadn't seen this post.
I must tell U that U need to brush up Ur measurement technics although
U are good on programming PIC.
a) Diode rectified output voltage of attenuated mains will be highly
non-linear as U are trying to convert about 1-2 volts signal and diode drop
is typical 0.6 Volts. Simple remedy is :- use diode directly on mains and then
attenuate the DC voltage. This will give somewhat better accuracy.
b) better use a small OPAMP based precision detector. This will give U best
accuracy at very little extra cost.
c) Avoid transformerless power supply. This will help in measuring multiple
inputs. Also instead of measuring RY-YB-BR fashion measure the voltages
at star point (neutral) and corrolate to line voltage. This way U can arrange
voltage divider network in star fashion and measure in cyclic way.
d) U can use Exxar or NS made voltage to frequency converters and transmit the results by optocoupler to RS232. Those have about 1-2% accuracy which is good enough. If U can't get them then send me private
email and I will show U how to get result from even 555 !

regards.
dayama