Opencart, OC 1.5.1.3, captcha error

OC 1.5.1.3, the Captcha image doesn't show on none of these pages:

  1. product / review section
  2. contact page
  3. by accessing this http://www.directmall.co.uk/index.php?route=information/contact/captcha (the direct link which should generate the image)

I can't see any errors (Apache logs / error.txt file). I can't see any spaces within the language files - in fact I've redownloaded the entire EN package just to make sure..

I suspect a broken dependency (even if I have GD.. there must be something else..); disabled entirely caching - need assistance!

Back in 2009 I've found trails of such errors on forums but it seems there was a language-file problem, meaning trails of spaces were sending the page headers earlier than normal - but I've checked most of the files I thought to be involved and I've cleaned all the extra spaces - with no result.

Thanks, Bogdan