I am about to use a recently viewed tv recipe for Rhubarb ketchup. Official ingredients include white wine vinegar but does say any vinegar 'will do'. I have some cider vinegar, will this dramatically alter the flavour or sweetness? Might I have to alter sugar quantity?
Cider vinegar is generally a little bit sweeter and not quite as sharp as white wine vinegar. With the other flavors you're adding, the difference isn't likely to be major. A good approach might be to hold back about a quarter of the sugar in the recipe, taste, and adjust to preference.
One thing in the cider vinegar's favor is that most of the acid present in the rhubarb is malic acid, the same compound that gives green apples their tartness. Apple and rhubarb typically get along quite nicely and I see no reason why they wouldn't here as well.