There were a lot more than Royal Marines stationed in Chatham where there wer a number of different barracks. In 1841it was the depot of 2nd, 3rd, 4th, 6th, 9th, 17th, 18th, 21st, 22nd, 26th, 28th, 33rd and another 13 Regiments of Foot. There would have been small numbers of men from all of those there.
MaxD