1. A method of detecting the presence of melanin in the dermis of an epithelial tissue having a dermis and an epidermis, the method comprising: obtaining an image of epithelial tissue to be analysed; processing said obtained image to determine from said image, data representative of a distribution of blood and melanin within said tissue; utilising said determined distribution of blood and melanin and a model of the propagation of light through epithelial tissue to generate image data representative of the expected appearance of epithelial tissue containing said determined distribution of blood and melanin wherein said determined distribution of melanin is assumed to be contained solely within the epidermis of said epithelial tissue; and detecting the presence of dermal melanin by identifying portions of said generated image which differ from corresponding portions of said obtained image by an amount greater than an amount due to expected variations in appearance due to variations in illumination and surface geometry, wherein detecting the presence of dermal melanin comprises: determining for portions of said image the ratio of image data for at least one colour channel of image data in said generated image relative to the ratio of the image data for said at least one colour channel of image data in said obtained image; determining an average ratio value for the image data for said at least one colour channel of image data in said generated image relative to the image data for said at least one colour channel of image data in said obtained image; and identifying portions of said image where said ratio of image data for said at least one colour channel of image data in said generated image relative to the image data for said at least one colour channel of image data in said obtained image exceeds said average value by a threshold.