Sad news Stuart! I have a Canon 400 too and yes I do have spots like yours. Lens, camera looks clean...coming home from day trip find nearly all my shots has a spots on a same place...very very anoying.

Danny thanks for an additional info on how to clean a sensor  at home. I so scared to touch it but one day I will  "have to close my eyes" and do it.

seeya  Tunde