Looking on the IGI...I easily found two other children of John Clammasson Snr. and wife Harriet, they were:
Thomas b. Jan 10/1825 ch. Mar 17/1825
Harriet Frances ch. July 28/1833
both at St Margaret's Rochester Kent
But as you say,there is no John...could John Jr. have taken his father's name after John Snrs. death? I have seen that happen..tho' birthdate of Thomas a bit late.