This Government website is a good place to start your pension planning. You can ask the Pension Service for an estimate - which they base on your contributions.
http://www.thepensionservice.gov.uk/approachin gretirement/home.asp
A couple of basic things: -
One gets one's entitlement to the Basic State Pension through minimum NI contributions paid over the years. If one doesn't have enough contribution years, one gets a proportion of the Basic State Pension.
If you had a company pension scheme at any time during your working career, that pension will provide you additional pension. The Government's pension service does not hold the records for this - you need to be talking to the pension trustees. If you were a civil servant, the Government's pension scheme for such staff is very similar to a private scheme - but run by the Government for it's employees.
If you did not have a company pension scheme (or were not a Government employee) you may have accrued an additional pension above the Basic State one. This has been called numerous things over the years - Graduated Pension way back in the 1960s and 70s. Then SERPS - State Earnings Related Pension, then the Second State Pension. The Pensions Service should have records of this 'block' of contricutions - and can provide an estaimate including them.