SERPS
Back in the old days the service opted out of the state pension system, we all thought, and were encouraged to think, you just lost the increased element earned by higher pay levels and paid a lower NIC, WRONG.
That's why it was called State Earning Related Pensions System. Only when you reached retirement age did you discover that you earned no state pension credit for those years and because you were no longer earning you could not buy them back.
My state pension is reduced by £95 per week for the lost years of contributions but naturaly I am happy to know it helps the service meet the deffence need