I figured it out using UnitySendMessage - in Xcode, in "AppController.mm", in -(BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation I added:

NSString *theOrientation = [NSString stringWithFormat:@"%i", interfaceOrientation];
UnitySendMessage("Game Manager", "DeviceRotated", [theOrientation UTF8String]);

and in Unity, in the script on my Game Manager gameObject I added -

public void DeviceRotated(string theMessage) {

    // do something here which needs to be done when device orientation changes

}

Being new to Unity I hadn't realised there could be 2-way communication between Unity & the Obj-c stuff. Probably elementary to most people here, but I thought I'd post it in case it helps someone avoid the troubles I had.