Fred Harvey's comments says it all really. It's a bit of a film noir, given that three of the four main protagonists are unlikeable, up against it and with little future (and this includes Jack Hawkins, who at least starts the film off with a stiff upper lip but then degenerates). The feeling of pessimism was accentuated by the photography itself appearing dark, and seeing it on TV didn't do justice to some of the panoramic outside shots.
It wasn't all that clear what distance Hawkins had to travel at the beginning of the film to escape the British, but despite looking very tired he made it to civilisation; so it wasn't entirely convincing that he needed to mount an expedition costing more than £200 and necessitating an ox-drawn wagon to retrieve the diamonds; one would have thought that having made it one way in a distressed condition he could have made it back by himself, with just a couple of pack mules for his provisions - but then he wouldn't have needed to recruit his disparate accomplices to fall out with, so there would have been less of a plot.
