Over time there have been two main changes affecting the rules on this.
The first came in 1978 when the 'state earnings-related pension scheme' was introduced.
SERPS offered a salary-related retirement pension to all workers via the state pension system.
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But there was a risk that for those in company pensions already this would amount to ‘double provision’ – a worker would be paying in to his or her company pension as well as paying National Insurance contributions towards an earnings-related pension from the state.