Well, I've managed to hold on to some Keynote documents now for FOUR WHOLE DAYS.
I have two macs - one at work, one at home; both accessing iCloud. I have an iPad and iPhone. I live on the iPad for teaching, but having watching Apple's videos where it all "just works" I registered all devices with 'Documents and Data' on iCloud. And of course it doesn't just work.
On Saturday I reformatted my iPad, as I mentioned in an early post, to no avail. In the past I have been able to UPLOAD documents from my iPhone, but they have never come down from the cloud to that device. So, on Saturday, I turned off 'Documents and Data' on my iPhone.
Since then, no deletions, although I haven't added any files either, which is also, I suspect, a trigger for deletions.
The long and the short of this is that the error is definitely a synchronisation error. Apple, PLEASE RESPOND!