Answer
If you use Hive's SmartCharge or FreeCharge modes, you can see all the charging credit you have earned right in the Hive App on the Charging Summary screens and on individual transactions in the Charging History screens.
Your earned credit will be aggregated each month and then applied to your paired British Gas Electricity statement. When your energy bill is produced, you will see credits added to your account for the amounts showing in your Hive App.