I cannot find any forum for ArcGIS Desktop so I'm posting in here as I'm not sure where else to post this.
I created a data driven page with a Spatial Reference scale field and the extent set to a data driven scale.
As soon as I did that the rotation of my map changed. It took some time but eventually after reading the following post: Data Driven Pages - spatial reference changes mid-way!, I worked out that the projection applied to be data frame had changed from British National Grid to some other projection.
Is this a bug that has since been fixed or just a random issue that more than likely only affects my MXD?