1. A method of encoding a set of texture data elements used in a graphics processing system, the method comprising: determining by processing circuitry a lower resolution version of the set of texture data elements; determining by processing circuitry one or more predefined blocks of difference values from a set of plural predefined reference difference value blocks, the determined one or more predefined blocks of difference values representing a full resolution set of difference data, the full resolution set of difference data representing a difference between: (i) an upscaled version of the determined lower resolution version of the set of texture data elements; and (ii) the set of texture data elements; and the method further comprising: encoding by processing circuitry the set of texture data elements as one or more encoded blocks of texture data representing the texture data elements of the set of texture data elements, each such encoded block of texture data: encoding a sub-set of texture data elements from the set of texture data elements and including: data indicating a base data value for the respective sub-set of texture data elements taken from the determined lower resolution version of the set of texture data elements; and an index value or values indicating a predefined block or blocks of difference values from the determined one or more predefined blocks of difference values representing the full resolution set of difference data representing the difference between: (i) the upscaled version of the determined lower resolution version of the set of texture data elements, and (ii) the set of texture data elements, the indicated predefined block or blocks of difference values to be used when decoding the encoded block of texture data to generate the data values to be used for the respective sub-set of texture data elements that the encoded block of texture data encodes; and storing by processing circuitry the one or more encoded blocks of texture data in memory for use by the graphics processing system.