Accurate surgery duration estimation is essential for efficient use of hospital operating theatres and the scheduling of elective patients. This study focuses on analysing the performance of previously developed surgery duration prediction algorithms at a specialty level to gain further insight on their performance. We also evaluate algorithm performance after applying filtering to exclude unreliable data from modelling, and develop and validate new ensemble approaches for prediction. These are shown to significantly improve the prediction accuracy of the algorithms. Employing filtered data delivers a reduction in overall prediction error of 44% (Mean Absolute Percentage Error from 0.68 to 0.38) employing the Random Forests algorithm, while using the newly developed ensemble approach delivers a Mean Absolute Percentage Error of 0.31, a reduction of 55% when compared to the original error, and a reduction of 18% when compared to the Random Forests performance on filtered data.