This is one of those stoopid things you trip over every now and then. Ever since I started with the CS5 upgrade Photoshop wouldn’t allow me to use a zoom marquee… instead it does a strange thing called “scrubby zoom“. This new feature means that when clicking and dragging the zoom tool your image will always zoom in towards the point you first click on. I’m sure this makes more sense to new users but nearly 20 years’ of muscle memory is very hard to shake!
Thanks to my colleague Aleksej for finding the answer for me. Just uncheck “scrubby zoom” in the contextual menu at the top of your Photoshop window!