Tree View - cannot edit check box

[GrahamLabdon]

Hi
I am developing a tree widget and have created a class for the tree items and a class for the model.
I can add items to the model and have them displayed in the tree view widget.
Each item has a check box and this is displayed, However, I cannot make the check box change value.
I have attached my source and would be grateful if someone could tell me what I ahve done wrong

Thanks

Graham