I've used Nikwax not on Carradice but on my Berghaus mountain jacket, which is just nylon canvas with a layer of Goretex on the inside. The way it works best is to wash the jacket in the washing machine, put it through the dryer at the approved temperature on the label, and to apply the Nikwax in a light layer when it is bone-dry. Just the hot-air drying already improves the waterproofing, the Nikwax improves the waterproofing further -- but also makes the jacket hotter because it interferes with the passing through of moisture. On the whole, I prefer just the washing and hot-air drying as more comfortable but that is not a full day's waterproofing, while the Nikwax lasts longer.
That sort of slight fluffiness on the weather side of tightly woven nylon (or whatever polypropylene) is apparently a major part of waterproofing with breathability, according to a plastics specialist* who once climbed one of the Irish mountains with us.
* He came with us only once because he was an Englishman over here on a local consulting job but I remember him because he told a really good joke. He said of a sleepy town we drove through on the way to the mountain, "Dunmanway is a town so dull, at six o'clock every evening the council rolls up the pavement."
