Thank you so much, yes that was what were confusing me even more.
Do you know if the docuement settings, should be 476 x 253 mm (or 438 x 215 mm?)
I've looked at their website page and the directions are a little murky, but I think I would set up at 438 x 215 (and specify some margin you are comfortable with, say 3mm to act as a "safe zone" where anything outside the margin may be invisble in the finished book) then add a bleed of 19mm and extend all art to the bleed line.
Though they say to set the offset for bleed marks to 5 mm I would check with them about that. 5 mm will leave the marks visible in the area that is folded over inside the cover and may be covered by the endpapers.