Hi tereshenkov, thank you for getting back to me.
My field datatype is Double. The table is stored in a file geodatabase. I did exactly as you said, created a new field (type Double) and used field caculator to round the field value. Nothing changed. I have attached a print screen so you can see exactly what I did, as well as my field values (original values 'SpeciesID', calculated values 'SpeciesID3). I re-ran the field calculator with another formular to make sure it was working and it does. My Regional settings are English (Australia) and under 'Customize Regional Options' my decimal symbol is "."
Any other thoughts? Cheers, Damian