> lampreys and peaches

The writers of "1066 and all that" enjoyed a yarn and the surfeit story was a good one. Whether heart disease or dysentery were the real reasons, the sources of the day lacked modern medical expertise, so diagnoses were, at best sketchy.

King John:
When John came to the throne, he lost his temper and flung himself on the floor, foaming at the mouth and biting the rushes. He was thus a Bad King....
John was so bad that the Pope decided to put the whole country under an Interdict, i.e. he gave orders that no-one was to be born or die or marry (except in Church porches)....
John finally demonstrated his utter incompetence by losing the Crown and all his clothes in the wash and then dying of a surfeit of peaches and no cider; thus his awful reign came to an end."
1066 and all that, Sellar and Yeatman 1930

There is a discussion on the sources for the story here as well as reference to Shakespeare’s King John:
http://www.ku.edu/~medieval/melcher/matthias/t54/0135.html

Henry I:
The lampreys could have been sea lampreys, returning to fresh water to spawn and die or river or brook lampreys. Here is a site discussing the lifecycle of the sea lamprey
http://ourworld.compuserve.com/homepages/BMLSS/lamprey2.htm

The lamprey story was first raised by Henry of Huntingdon in his book "The History of the English People" 1123(ish) where he also tells the story of Cnut(or Canute) trying to stop the waves.
http://www.oup.co.uk/worldsclassics/mag/eels/

Here's a good site for Bill on medicine and Royal deaths in Britain:
http://www.doctorzebra.com/prez/a_royalhx.htm