Used NCS and can't trace document using the reference No

Hello,

Not sure if anyone has experienced this in the past. I used the NCS to apply for Naturilization and was given a special delivery tracking ref No. I have tried several times to track the mail but have been unsuccessful. Spoke with royal mail and they confirmed they can't track it because its not on their system. When to the office and they assured me all is fine and that my application must have reached HO on next day. The issue is that my code starts with SC and this is not common code on royal mail website.

Any help will be appreciated