I've found a tomato sauce substitute! I wasn't overly happy with the quick dextrose version so in my search I came across Doritos Salsa (the kind in the chips/corn chips section of the supermarket). Yes it has sugar in the ingredients, but only contains 2.8g sugar per 100g, and some of that will be from the tomatoes themselves. Perfect for those things where they wouldn't be the same without some tomato-ey goodness :)