no..not quite correct! In a lot of Natural Causes Deaths, Coroners clearly state the Cause of Death is not known particularly where there is no evidence of wounds, marks, scrapes, fractures, bruising to the body or internal organs and bleeding in the brain etc.
The Coroner can't say heart failure, particularly if the heart muscle is healthy and no signs of previous Ischaemia or Atherosclerosis not greater than 70%. (If the family presents evidence of Medical History outlining treatment received, this may pursuade him/her to make a finding.
The Coroner opens the inquest the moment he signs the Order for Post Mortem, he then adjourns the Inquest immediately awaiting the Pathologist's Report and Police witness statements. If the Coroner finds further compelling evidence he will send the case back to UK's CID for further investigation, as i have already mentioned.