Hi Davina,
Please contact the National Insurance helpline so they can investigate the accompanying letter/cheque you have sent in and advise.
If you have access to a Personal tax account, you can also use the webchat services.
National Insurance: general enquiries
Number - 0300 200 3500
Overseas - +44 191 203 7010
Open - Monday to Friday: 8am to 6pm, Closed on Saturdays, Sundays and Bank Holidays.
Thank you.