Place the chicken breasts into the bottom of a slow cooker. In a bowl, stir together the tomato puree, mustard, lemon juice, garlic powder, maple syrup or golden syrup, Worcestershire sauce, chilli powder, cayenne pepper and tabasco until the mixture is well blended.
Pour the sauce over the chicken, set the cooker to Low, and cook for 6 hours. Shred the chicken with two forks, and cook for 30 more minutes.
For best results, after 6 hours of cooking, make another batch of the BBQ sauce, pour over grated chicken and stir; cook another 1/2 hour until finished.