The general election of 2010 saw 59/59 constituencies in Scotland sent an MP to the Union Parliament (to be part of the Union)
 
Only a general election can allow the electorate to reconsider (by voting between one candidate and another) their decision of 2010. A referendum is a denial of the electors parliamentary authority and opens the question – by what authority do the 650 MP’s today sit and legislate in the House of commons if it is a referendum that now determines membership and authority to make laws?
 
The electors of Witney or Doncaster North have not had a referendum to determine wether their MP should (or not) sit in the House of Commons.
 
If Mr Cameron "supports the Union so strongly " then he and only he can call a General Election and allow electors to exercise their authority and reconsider their decision of 2010.