I think I have found a mixer! (The Kenwood recommended here.) PLUS, I found cocoa at my Oxfam as well! And THEN it mysteriously appeared at the Tesco Metro near us--but the Oxfam was fairtrade, for a good cause, and cheaper, so I went with them in the end.
Thanks so much! My DH would thank you as well if he knew all of this work was for his birthday cake!