If you know how much your annual pension is then use this link, be sure to tick the box saying that you don't pay NI on it, it will tell you what your monthly net amount will be, is bang on for mine to the penny:
Income tax is taken from all income (including pensions). Your pension payer will need to know your NI number and PAYE code. National insurance is only paid by employees (and their employers). So if you are retired etc, and your only income is from pensions etc, you should not pay NI. If you have any problems see your local Citizens' Advice.