I wonder if it could be some anthraquinones from the diet or from an herbal laxative.
In the US before 1999, phenolthalein was used as the active ingredient in Ex-Lax. I'm not sure if phenolphthalein is still in use in the UK (Australia?) but if it is and you used it, it can find it's way into the urine.
Same goes for Correctol or bisacodyl. This structure is farily easily hydrolyzed and/or oxidized to a hydroquinone.