Thanks chaps - no problem with space - 78 GB out of 250 free. Don't think it was the software update - it's happened a few times and no sign of any of the update checking routine running. I had done the disk permissions but I've done that again. Wasn't sure that the safe boot would work as I have a wireless keyboard but it did. (I assumed that the OS would have to be running before the keyboard was recognised/paired).
I've just done a manual update out of phase of the autoupdate and it's found the 10.5.7 - mine reports only 286 Mb. Downloading now.