NIC 3 Online Payment does not define which similar years you want to pay off

I need to pay off some years to increase my state pension but it does not allow me to specify which years when paying online. The amount to be paid for some years are exactly the same (i.e. £824.20), so how do I know you will only credit the years I want to be paid. This is important as some years costing £824.20 will not increase my pension and these would be wasted payments. The only way I can comfortably see this being achieved is by sending a cheque specifying which years to pay - is this correct. Last time we sent a cheque payment it took 5 months for it to be processed and I take my state pension in 2 months time.

Hi Minton,
In order for the payment to be allocated correctly, an 18 digit reference number will need to be generated by HMRC.
To get this, you would contact the National Insurance Department on 03002003500.
When the advisor generates this, they will make note of the years the payment is for on your account.
Thank you.