The British Army Worldwide Index 1851 has only one William Dennis stationed in Chatham. This man served with the 1st Battalion 60th ( King's Royal Rifle Corps ). the service number is missing from the record. In the British Army Discharges , 60th Foot , William Dennis , service number 2973 , of 60th Royal Rifle Corps is listed as died in Delhi 14 Sep 1857. He is also listed as " Died of Wounds " in British Casualties of the Indian Mutiny. These records are all on FindMyPast. There doesn't seem to be a death record in the Armed Forces Deaths. Could be your man.