It's a terrific deal, too. No investment can match it, because you can get a brilliant return on your money. Yet voluntarily buying more state pension isn’t right for everybody.

The state pension is not given out as an automatic right, everyone needs to build entitlement through their working lifetime by making qualifying National Insurance (NI) contributions.

Under the new state pension, paid to those retiring from April 6, 2016, those with less than 10 years of NI get nothing at all. Thirty-five years of NI secures the maximum amount.

The state pension may not bring riches but thanks to the triple lock it will increase by 8.5 percent in April, to a maximum of £11,502.40. It pays to get all you can.

If you have yet to retire, your first step is to request a state pension forecast online at Gov.uk/check-state-pension, said Andrew Tully, technical services director at Nucleus Financial. “This will show how much you are likely to get, when you will get it and your options to build up more state pension.”

Alternatively, you can complete and post the BR19 application form or call the Future Pension Centre on 0800 731 0175.

Most of us build state pension entitlement automatically through PAYE or a self-assessment tax return.

Some end up with gaps because of illness, disability, unemployment or raising a family. The self-employed can miss out if they didn’t make enough profits to pay NI.

In most cases they automatically get free NI credits. If unsure, call the NI helpline on 0300 200 3500.

If you cannot claim any NI credits you can quite literally buy extra state pension instead, by making Class 3 voluntary NI contributions, said Stephen Lowe, retirement specialist at Just Group. “Each additional qualifying year gives you 1/35th of new state pension, worth £328.64 a year from April.”

The cost depends on which years you need to buy. Buying one extra year of state pension costs £907.40 for the current tax year. This falls to £824.20 for the 2022/23 tax year, £800.80 for 2021/22 and £795.60 for 2020/21.

Your payments could get a brilliant return.

Someone topping up the last 10 years of NI would pay a total of £8,772.40, Lowe calculates. Based on next year’s state pension, the would get £3,286.40 extra income in the first year alone.

This means that they will be in profit within just three years.

Over a typical 20-year retirement, that £8,772.40 payment would have bought them extra state pension of £65,728, Lowe said. "In practice, it is likely to be worth even more, as the state pension should rise over time.”

So buying one extra year of state pension for £907.40 today could give you an extra £6,572 over 20 years of retirement.

This looks terrific value but this may work better for some than others. “The healthier you are and the higher your life expectancy, the better your return is likely to be. It may not work as well for those in poor health,” Lowe said.

If you die before state pension age, and get nothing in return for your voluntary contributions. And if you only live for a few years after retiring, again, you won't get such a great return.

READ MORE: Martin Lewis calls on Britons to check if they can boost state pension

It is possible to make voluntary contributions after you hit state pension age. However, these can only cover years you missed while of working age.

Typically, you can make voluntary contributions for the past six missing years.

However, under special rules people can currently top up missing state pension going all the way back to the 2006/07. This applies to men born after April 5, 1951 and women born after April 5, 1953.

Claims originally had to be made by last July but the tight deadline led to jammed HMRC helplines, forcing the government to extend the deadline to April 5, 2025. That may seem far away but given the huge claims backlog, there is no time to waste.

This is a once-in-a-lifetime opportunity to boost your state pension and get a great return on your money, so don’t waste it. Just be sure that you really do have a gap to plug.