At last, a solution. Turns out there were some other variations of the cover kicking around - I guess the artist must supply several different potential versions of a cover design, from which the editorial people pick what they think might work, then ask me my opinion.
I've now seen yet another design for the book (all mild variations on each other, I should point out, rather than anything radically different) which fits the bill exactly. It seems to make the marketing people happy, and it certainly makes me happy. It's like the cover I'd already seen and approved, but better (basically, a different bloke in the foreground, but similar pose). So, sorted, really. Now I can get on with some other stuff, like ... I don't know ... writing, or something.