"Once upon a time" I knew all this stuff, but memory has faded. I do remember where to look to find it though.
Also "Once upon a time" we had the pension advisiory service, TPAS, but that was combined into the Money and Pension Advisory Service, MAPS which is wholly independent and government funded. As others have said, their 'user' website is moneyhelper.org.uk.
This page is particularly useful, scroll down and it tells you a bit about GPS, SERPS and so on.
https://www.moneyhelper.org.uk/en/p...state-pension-work-and-how-much-might-you-get
This short and specific page covers serps contracting out.
https://www.gov.uk/contracted-out
Contracting out didn't always end your NI contributions, for some periods it just reduced them.
Youcan see historic rates back to 1999 here:
https://www.gov.uk/government/statistics/main-features-of-national-insurance-contributions
If you were working for an employer which had a defined benefit pension scheme, also called final salary or sometimes career average scheme, it is likely you were contracted out of SERPS, but that has no effect on your basic state pension. In return for the lower employer and employee NI contributions from contracting out the pension scheme had to meet certain minimum standards, originally the guaranteed minimum pension but later the reference scheme test. So contracting out wasn't an easy ride for employers, it just gave them some help with the SERPS was there so that people who had decent earnings but didn't work for a big employer, or didn't choose to contract out and start a personal pension, could accrue something more than the basic state pension. Again though, whether you were contracted in or out had no impact on basic state pension. But it does, after Osborne announced muich higher basic pension paymnets from 2016? there was a quiet caveat - not in his announcement. If you had contracted out you won't get the full new higher rate pensiuon he boasted about. And the indexation of GMP in payment stopped.
All this is a minefield. You can call the moneyhelper people, and if you have not yet reached state pension age you can - should even - get a pension forecast. It used to be a form - BR19 - but now you can do it through the Government gateway. It will identify the elements - basic, serps, gps - and a total.
https://www.gov.uk/check-state-pension
It's all a lot simpler now, not necessairly better, but those of us of certain age still have a mish mash of historic benefits.
Take care - there are many sharks swimming in the sea who are only too happy to take your money for advice or management. Make sure you work through the moneyhelper site or the officlal governemnt site, not any imitations. If after getting the basic facts you then choose to pay for advice thats up to you, but start simple and start with the ones suggested.