[lolyton]

Sorry, i'm a little late on this one, and was reading what Steve had to say about calibrating your monitor to Photobox's output settings. You said to begin with that you are very happy with your colour prints, which would suggest that the colour calibration may not be too far out. Also....and this is the bit i am confused about, if you are sending them a B+W image to print, i am assuming it is in Greyscale (if it is not this may go a long way to explaning your problem), the calibration of your monitor will have no effect on a cast on the prints (i am ignoring brightness and contrast and focusing solely on colour as i am assuming it is a colour cast problem). A greyscale image will be printed in greyscale (using only tones of black ink), thus resulting in an image without a cast. The only area a cast can occur (as far as i am aware) if printing in greyscale would be from the type of paper used (i used to print B+W on some epson papers and they used to have a green cast to them).
Please correct me if i'm wrong (which i normally am).

I would suggest you ensure that the images are converted to Greyscale before sending them to be printed (providing this does not effect the image in anyway) just in case this is the reason your cast has been caused. There are so many ways of converting images now that sometimes you may still have a small amount of colour registering (maybe).