Thanks for the fast response, fellow mancunian too 
So to use this sensor to detect different coloured lines on paper, I'd basically want three of the Sneak Thief circuit, one for each colour photodiode?
Looking at the data sheet, the different colours have different sensitivities to light. Is this something you would recommend I try and address in hardware, with different value resistors on each colour or in code with something to give a calibrated RGB value?