Waiting to receive supporting documents

I received my passport earlier this week and my documents (marriage certificate) came by Royal Mail the following day. I didn't pay the extra for secure delivery.

Return of supporting documents by secure delivery. Can it be tracked? Using what tracking number?

We applied for two Child First Passports, both sets of documents arrived one day ahead of the Passports. The applications were approved on Friday and the documents arrived by snail Royal mail on Tuesday. We did not use special delivery, but live in London which may help!