Ellie - just found this on the slimming world thread:
 Irn Bru Chicken is simple : 
i used 3 chicken breasts, 
1 can diet irn bru
same amount of passata
splash worcestshire sauce
splash balsamic vinegar
1 tbsp minced garlic
slow cooker for 4-6 hours, with lid off for last hour 
What do you think? Dare I try it?