It's happened to me twice!
Below is a rough guide to what i did, sorry for the lack of detail and photos. (Done this a month ago and only just got around to posting)
Broken Clamp

-I removed the small broken part which the clamp bolt goes through
-I found a Jubilee clip and cut a section out of it
-Size it up and bend it so that it fits the contour of the clamp
-Angle the end up

-Then drill the hole in the angled up end
-Find a course threaded short self tapping screw
-Drill a hole near the other end of the bent jubilee clamp for the screw to go through
-Place the new clamp in position and make sure its level with the edge (See picture below)
-Mark the plastic in the newly drilled hole
-Make a small pilot hole in the plastic slightly smaller than the screw being used to attach the clamp
-Fix the clamp in place with the screw and voila!

Hope this helpful to some of you, if there are errors or steps which you think should be added then let me know and i can update.
Im also aware from seeing one of the most recent mini magazines that you can now buy the new stalks from minispares i believe? There around £40 for the side with the clamp.