I have another question regarding Time Machine problems (Time Machine always makes a full backup), which I think I have resolved. As soon as I am sure, I will post my solution there.
I am now getting the sort of behaviour I expect from Time Machine (proper incremental backups) but it seems to skip some backups.
I found this morning, that it had not backed up overnight, and I found this afternoon, when I got home from work, that it had backed up this morning, but not during the day.
The iMac was put to sleep, and, obviously, there was no activity to warrant more backups. However, the control panel does say the it keeps “Hourly backups for the past 24 hours”, and that’s clearly not what’s happening.
The question is, is this the correct behaviour of Time Machine: only making backups when the machine has been busy? This would imply that the message is not quite correct.