Hi Kaveman
Sorry I haven't told you how I got on, as I promised. It's because I haven't reinstalled OSX yet because I have been shutting down each night rather than putting the machine to sleep. This seems to have solved my immediate problem as the spinning ball seldom appears now.
Looking at the activity monitor, the green free sector of the system memory gets smaller whilst the blue inactive sector grows throughout the day to the point where it is less than 1GB of my 4GB of memory. If the spinning ball is going to appear it is at this time of day with the smallest amount of free space available that it shows. When I turn on in the morning, after all my usual applications have opened, there is about half of the disc free.
Phil