Know this: mixing by hand is pretty useless unless it's a really small batch. Those who say you can't get a proper result using an electrical mixer, probably used a regular high-RPM drill and some cheap paddle, instead of a proper low-RPM mortar mixer and a proper grout paddle.
In fact, you're more likely to get a bad result when you mix by hand, than you are when using a proper mixer. The grout is supposed to be completely homogenous, and there can't be any lumps. By using a proper grout paddle and a low-RPM mortar mixer, you'll avoid mixing air into it.
Im trying to find something for this application rather than using my lovely new dewalt, any suggestions of where i can get a decent mixer and paddle for my grout?
Cheers