I had already tried this and it didn’t seem to work. Using the data in the original post, ie. PLA999, PLA1000, PLA1001, the substring results (hidden column) display correctly as 999, 1000, 1001. Sorting on this data in descending order gives 999, 1001, 1000.
Was wondering if the results of the substring were still alphanumeric and causing a problem.