How to calculate the net salary?

[londonflier]

Hi
I have been offered a job at 50K. I would like to know how to calculate the net salary per month.
My tax code is 543L and I would pay a pension contribution of 5%.
Where do you take the pension off? Off the taxable income?
I used these 2 websites but the come with different figures

http://listentotaxman.com/

http://www.thesalarycalculator.co.uk/

Moreover I would like to know if there any difference in the monthly net in case they offer me a basic 47K+3K allowance, or a basic 50K.

Thanks in advance for all your tips.