The first thing to acknowledge is that the constituency of North and South Esk is the author's creation. However, you don't need to be Mungo Park to work out that there must be some correlation between North and South Esk and the real world, Edinburgh being a real place, and 'south and east of Edinburgh' being a vaguely definable geographical area.
In fact, North and South Esk bears some resemblance to the Midlothian parliamentary constituency – prior to 1983's Boundary Commission changes – but also bites a small southernmost chunk out of the present Edinburgh Pentlands constituency and a westerly chunk out of East Lothian constituency.
Gregor Jack, too, is fiction, and bears no resemblance to any MP.
Thanks are due to the following for their inestimable help: Alex Eadie, who was until his retirement the MP for Midlothian; John Home Robertson MP; Professor Busuttil, Regius Professor of Forensic Medicine, University of Edinburgh; Lothian and Borders Police; City of Edinburgh Police; the staff of the Edinburgh Room, Edinburgh Central Library; the staff of the National Library of Scotland; staff and customers of Sandy Bell's, the Oxford Bar, Mather's (West End), dark's Bar and the Green Tree.