well yes, but I don't know how or why
The root directory was full meaning that you can only get into the system via the terminal (what you actually get is a mesage saying that Gnome power manager is not installed correctly and you cannot then log into the desk top but can only get to the terminal using Ctrl-Alt-F1). There is no warning of this critical state approaching.
The key seems to be to free up space on the root directory by either moving or deleting files, but this did not seem to work. In the end I recall using a command which automatically gives a % increase in the size of the root directory but I cannot now trace what that was for future reference. So the key questions are: how do you stop this happening in the future and if it does happen what is a quick solution to the problem? (a trawl of the internet offers no clear solution).