1. A data processing apparatus for performing a transform between spatial and frequency domains when processing video data, the data processing apparatus comprising: transform circuitry configured to receive N input values and to perform a sequence of operations to generate N output values representing the transform of said N input values between the spatial and frequency domains; a base circuitry configured to receive M internal input values generated by the transform circuitry, where M is greater than or equal to 4, and to perform a base operation equivalent to matrix multiplication of said M internal input values by a matrix comprising an array of coefficients c and having the form c 0 c 1 c 2 … c M - 1 c 1 c 2 c 3 … c M c 2 c 3 c 4 … c M + 1 … c M - 1 c M c M + 1 … c 2 ⁢ ⁢ M - 2 in order to generate M internal output values for returning to the transform circuitry; and the transform circuitry being arranged during performance of said sequence of operations to generate from the N input values multiple sets of said M internal input values, to provide each set of M internal input values to the base circuitry in order to cause multiple sets of said M internal output values to be produced, and to derive the N output values from said multiple sets of M internal output values.