I'll second that. When I had a Hotpoint Supermatic, supermarket own brand powders worked well in the wash and were the easiest to rinse out. With branded powders, it would often take five or six rinses until I felt the rinse water ran clear enough for my liking. High suds powders were even worse, and would not rinse out acceptably unless I refilled the wash tub for a deep rinse, negating all the time and water saving benefits a twin tub is supposed to provide.
Soap flakes work well, but AFAIK their production has been phased out now in favour of the liquid version. Worth a try, but unless you already have perfectly soft water, you will need to use a water softening agent in the wash in order to ensure a good lather and prevent scum forming. Adding a separate oxygen bleach will kick it up a notch too.