Heat the slow cooker to HIGH.
Place the chicken breasts into the bottom of the slow cooker.
Mix the ketchup, honey, mustard powder, water, sugar and vinegar together. Pour all over the chicken.
Place the lid on and cook for 2-3 hours (or 5-6 hours on LOW).
OPTIONAL: If the sauce is to thin, remove the chicken breast and set aside. Reduce the sauce down on the hob, by heating over a medium heat for a few minutes, until a spoon leaves a trail at the bottom of the pan (like in the video above). Add the chicken back into the sauce.
Shred the chicken and mix into sauce.
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