It is vital that you look at who wrote both primary and secondary sources.
You probably know from GCSE that primary sources may be heavily influenced by all manner of characteristics belonging to the author. For example, if they were a close friend of Charles I, they are unlikely to speak positively about the Civil War that led to his death, and are more likely to exaggerate or highlight the bad things that Charles’ opponents did.
The same applies for secondary sources written by historians, as they may be influenced by their characteristics and life experiences. We must also, however, look at the methodology of a historian and whether or not they are a member of a particular school of thought.
For A-level this particularly refers to Whig historians and Revisionists.
Whig historians have typically seen history as a continuing march of progress towards the ‘ideal’ circumstances of British democracy, constitutional monarchy and Protestantism. For them, the ‘purpose’ of the past was to 'create' the present. Revisionist historians challenge these interpretations.
It is very important to know whether the author of a source is Whig or Revisionist (or neither) because it will affect their writing. For example, a Whig historian is always likely to see the Protestant Reformation in a positive light because they see Protestantism as the ideal.