I found a solution from 'Eileen's Lounge'. The solution given was:
"The only instances that I have seen, where the taskbar refuses to hide, is when there is some notice or note from Microsoft advising that there is a new update or something similar. Then, until you do the wishes of the almighty MS, the icon will remain in control of the taskbar."
Well it wasn't MS but Dropbox saying I had to re-install to get synch working properly. Re-installed DB and the taskbar now hides.
So nothing to do with CS, just a Windows idiosyncrasy:D.