Make the ketchup: put all the ingredients in a bowl with 40ml water. Mix until the sugar dissolves, then spoon into a sterile glass jar or bottle. Let sit for at least 30 minutes before serving, to let the flavours develop.
Cook the bacon under a hot grill for 2-3 minutes on each side, until crisp.
Griddle the bread on both sides in a hot griddle pan. Make 4 sandwiches, spooning some ketchup over the bacon.
Cook's tip: The tomato ketchup will have a chance to develop a deeper flavour if you make it the day before. Keep it in a sealed jar in the fridge for up to seven days.