A method for generating and processing data for the display of a stream of video data on a display screen connected to data processing apparatus, said method comprising the steps of: selecting an MPEG compliant data stream of video data selected to be viewed by a user in a first format via said apparatus, the largest frames of said video data known as I frames; parsing the selected and received video data to extract the bit_rate and vbv_ delay values of the data field headers; setting a prefill threshold of a decoding buffer according to the extracted values; storing in the decoding buffer the selected normal data stream of video data; processing the stored video data to be viewed by a user in a first format; said user selecting to view the said video data in an altered format in the form of a trick mode stream of data, generated by the apparatus; and following the user selection of the altered format, the method includes the steps of flushing the data stored in the decoding buffer, waiting for the frame start code and determining the size of the first received I frame, estimating the required buffer memory size by adding a tolerance percentage value to the determined I frame size to provide a buffer memory size less than the vbv_buffer_size value which is provided in a header field transmitted with said video data stream, and setting the prefill buffer threshold of the decoding buffer according to the estimated buffer memory size, buffering the trick mode stream of data in said decoding buffer and processing the buffered trick mode stream of data when the buffer reaches the prefill buffer threshold and characterised in that the size of the said first received I frame is calculated using time stamp data, PTS, transmitted as part of the said video data and quantised in steps of one frame and, upon the said user selection occurring, a change in the time stamp data indicates that the payload of the next data packet refers to the next frame received and is used in the estimation of the value of the I frame data and hence the required prefill buffer threshold for the duration of the provision of the user selected trick mode stream of data. selecting an MPEG compliant data stream of video data selected to be viewed by a user in a first format via said apparatus, the largest frames of said video data known as I frames; parsing the selected and received video data to extract the bit_rate and vbv_ delay values of the data field headers; setting a prefill threshold of a decoding buffer according to the extracted values; storing in the decoding buffer the selected normal data stream of video data; processing the stored video data to be viewed by a user in a first format; said user selecting to view the said video data in an altered format in the form of a trick mode stream of data, generated by the apparatus; and following the user selection of the altered format, the method includes the steps of flushing the data stored in the decoding buffer, waiting for the frame start code and determining the size of the first received I frame, estimating the required buffer memory size by adding a tolerance percentage value to the determined I frame size to provide a buffer memory size less than the vbv_buffer_size value which is provided in a header field transmitted with said video data stream, and setting the prefill buffer threshold of the decoding buffer according to the estimated buffer memory size, buffering the trick mode stream of data in said decoding buffer and processing the buffered trick mode stream of data when the buffer reaches the prefill buffer threshold and characterised in that the size of the said first received I frame is calculated using time stamp data, PTS, transmitted as part of the said video data and quantised in steps of one frame and, upon the said user selection occurring, a change in the time stamp data indicates that the payload of the next data packet refers to the next frame received and is used in the estimation of the value of the I frame data and hence the required prefill buffer threshold for the duration of the provision of the user selected trick mode stream of data.