we eventually found out it was down to a php 5 upgrade on 2 of our servers (hence not the third) After searching for a fix which seems like forever I found this and it's cured
This is the code change I mentioned previously - saying that, there are a couple of other fixes for this issue, if it happens on your third server and this doesn't cure it let me know and I'll pass on what I know.
So, changes made. Those affected, please clear your cookies and restart browser.
If still happening, let us know.
JF