Well, I'm not quite sure how the communities work in AppleLand. I've had no reply from the community or from Apple. I've not experienced this level of disinterest on any other support site. Maybe my question was too boring....
As it happens, although I haven't resolved the problem as described, I've got Time Machine working again. I don't exactly know how, but I updated the firmware on the MyBookWorld to the latest, and stopped using the AirPort wireless and switched to a wired connection. Time Machine seemed to be able to find the external disk and recommence its backup regime.
If anyone cares to answer my original question that would be great. Google shows many sites with posts asking similar questions, but without any sensible, correct, user oriented answers.
@neil56 - I am having the same problem. Any file I try to delete from my WD won't delete. I get a popup that says "The operation can't be completed because the item 'iMovie Cache' is in use," or some variation of that.
I have searched everywhere for an answer and can't seem to find anything sensible. If you have a solution to the problem, can you please post it?