Charilaos
This is almost certainly a path issue. First off, open the file again in TextMate, and press "opt-cmd-," which opens up the LaTeX preferences. Then click on the "Verbose TeX output" option, and type "-shell-escape" without the quotes in the Options box.
In fact this was all that was needed, it now works perfectly :-)
Many thanks for this, aren't problems simple to solve when you know the answer, and indeed, how to ask the question properly.
Graham