I was given a link to this thread by an AppleCare representative.
I noticed that in the last few days we had been getting through significantly more data than normal - even overnight when the computers are relatively idle it was using about 7 GB.
I was able to log into my Draytek Vigor 2710 router and find out that it was the IP address of my Mac Pro that was using all the data.
Then I used Little Snitch Network Monitor to see what processes were using all the data. As mentioned above it was ubd - and more specifically it was 3 processes - p03-ubiquity.icloud.com, p03-content.icloud.com and another called something like blob. something .windows.net (apparently this is cloud storage used by iCloud).
Concurring with what was said above, I was told it was Documents and Data that seemed to be the problem. I signed out of iCloud and allowed it to delete everything from my Mac Pro, then signed in and restored everything except Documents and Data.
My plan is to see how things go without using iCloud Documents and Data - and if no further problems I'll try ticking that again and monitor it closely.
My suspicion is that some corruption occurred with iCloud documents or the sync process (second problem I've had with iCloud so far).
It's a shame because things like this NEVER seem to happen with Dropbox - even though I use Dropbox far more extensively and for far longer.