Does your stool contain blood? If so, it might be colon cancer.
Did you have diarrhea before you started this diet? It is most likely that your body is trying to detox (which is normal, because you have been living of sandwhiches, sausage rolls and crisps), so it is nothing to worry about. You might want to add in some fibers (eg. some fruit, although this is not allowed on the strict diet).
What are you doing to protect your liver?