@alex_malyu Thanks very much that's what I was missing.
To get it working I removed the include for DifficultyAnalyser in the SokoGenerator header file
#include "DifficultyAnalyser"
I simply the following in the SokoGenerator header
class DifficultyAnalyser;
DifficultyAnalyser* diffAnalyser;
I then included the DifficultyAnalyser header in the SokoGenerator cpp file, and created a new instance of it
#include "difficultyanalyser.h"
diffAnalyser = new DifficultyAnalyser;
And in the header for DifficultyAnalyser I included SokoGenerator
#include "SokoGenerator.h"
Thanks for your help!