Medicine Through Time - How use is this source type questions - Q3 of Edexcel 2013

My students get really worried when looking at "how useful is this source" type questions of the Edexcel medicine paper. This is currently Q3 of the 2013 onwards paper. This lesson seeks to build up their understanding of how to explain a source's usefulness. This lesson contains two example questions, some sentence starters to develop structure and most of a model answer. Students can then be set an example for homework. This lesson could be supported by pages 30-31 of the new edition of the Edexcel endorsed textbook (2013 edition). Comments are appreciated :-)