Voluntary Class 3 contributions code?

I seem to have missed something during Covid. When paying voluntary contributions from Canada in the past I simply made sure my National Insurance number was on the cheque and in any accompanying letter. Now it seems this is not enough. Do I need some additional code and, if so, how do I get one? Thanks

Hi Pittenweem Crail,

For overseas payments by cheque please write your NI number on the front of the cheque.  
This allows our banking services to process your payment to your NI record.
We are in the process of updating the payment information and apologies for any inconvenience caused.

Thank you.