Pain this one - but simply solved!

When you use the AutoFormat selection, you turn on an option that stops you from editing the shape again [UNLESS of course, you know how to turn it off ;-)]

Once you have selected the AutoFormat Style, you need to select the chart again by right-clicking and untick the 'Use AutoFormat' option - you won't lose the style that you have set, but you will unlock the current chart for editing!

Ta-Daaaa! - told you it was a quickie!