You can delete a booking by going to the same area where the booking was added.
Go to your Runs module, select the Run by clicking anywhere along that Run name:
On the next page, select the session:
From here, you will see all the bookings in that session.
From the list above, click anywhere along "Toby Smith" to open this booking:
Toby's booking is linked to his parent - so the details for the associated booking will also appear. You will only delete one booking - click on the little "bin" icon in the right corner of Toby's record. This will delete his booking and complete the process.