Like Janesy said, could be the roll-bar bushes, most likely their drop links (the links between the roll bar and the suspension struts). It sounds like it could be the rear ones, as I recently had the same problem with my Focus - one of the drop links actually broke up, and resulted in a clonking noise when going over bumps or cornering. It could also be the front drop links, but I had this problem at the beginning of the year and it was more noticeable when turning rather than driving normally.
Check your rear drop links first, I'd say that's the most likely culprit. They are easiest to see. Simply get down under the back of the car and look at the main cross member running between the wheels. The drop links drop from either end and are attached tot he rear roll bar with bushes. Ill attach a pic of what I found under my car when I had a look.
This is what I had on the passenger side, drop link intact:
And this is what had happened to the other side. The bolt had sheared off and the whole bottom part of the bush had fallen away:
Luckily, replacing these is fairly simple. I had both of mine done within the hour. All you have to do is use a ring spanner to hold the top nut, then use a socket wrench on the bottom to unscrew it. It's a long bolt, so takes awhile, but once undone, it should just drop right out. Then you can simply pull the bushing out from between the ARB and the subframe. Reverse the procedure to fit a new drop link; insert the bush, push the bolt through, and tighten the whole lot up. I bought replacement drop links from ebay, cos me 11 quid delivered. Just search for "Ford Focus rear drop links". You might be able to find more information in
this thread which I started on focusowners.com asking how to do it myself. Some more of my pics are in the OP.
Checking the front drop links is also easy. Jack the front and take the wheel off...you should see a long thin metal rod connecting the suspension strut to the ARB. At each end of this rod are ball joints with rubber bushes. These can perish and fail, causing knocking noises when turning. Again, replacement is technically easy; undo the nut at each end, remove and replace, and front drop links can be found on ebay for about £15 a pair. I don't have any pictures of when I did these, but if you have any questions, fire away.
