Use sp_drv_unitext_substring which accesses the Transact-SQL function substring.
sp_drv_unitext_substring locator, start_position, length
locator – a unitext_locator that references the data to manipulate.
start_position – an integer specifying the position of the first character to read and retrieve.
length – an integer specifying the number of characters to read.
None.
A column of type unitext containing the substring retrieved.
