Any clues?

When a flock of sulphur-crested cockatoos spent some time breakfasting on my pecan-nuts the other day I zoomed in on several birds and 'hoped for the best', as you do, when the subject isn't close enough to be sure you will end up with a good shot...

So, when I uploaded the photos to the computer, you can imagine my surprise when I found this bird among the flock! This bird doesn't have a crest that I can see and its eyes are not as 'beady' as the cockatoos.

At first, I thought perhaps this is what a young sulphur-crested cocky looks like, but it isn't...the young birds look just the same as the older ones, just smaller...

Can anyone identify this one for me? I've been Google searching it for days and have come up with nothing!