@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
I simply the following in the SokoGenerator header
I then included the DifficultyAnalyser header in the SokoGenerator cpp file, and created a new instance of it
diffAnalyser = new DifficultyAnalyser;
And in the header for DifficultyAnalyser I included SokoGenerator
Thanks for your help!