Hi Rob
As you may, or may not be aware, there is no birth certificate for a James Collins registered in the Medway district. The closest are as follows:
Births Dec 1838
COLLINS James Maidstone 5 278
Births Mar 1839
COLLINGS James Hollingbourne 5 241
Births Jun 1839
Collens James Tunbridge 5 433
Births Mar 1841
Collins James Cranbrook 5 77
Births Dec 1841
Collings James Malling 5 297
Births Dec 1842
COLLINS James Rye 7 398
Given the above, James doesn't look like he was baptised in the Chatham area. Another possibility is that he was illegitimate?
Regards
Lee