Ah, a glimmer of hope then....because I did only try two sets there's a good chance they could have been empty. The dog was marking the area rather than head down hole etc. The thing about the second set that makes me reluctant to write him off altogether, was the fact that when I put him down he ran through the hedge all the way along. He ain't an energetic ferret, and is clingy, so to run off like that was a bit wierd. I'm hoping he was following a scent through the hedge.
What I'm tempted to do is to get up early, let the dog chase the rabbits so I can see which warrens they go in,