After two failed in the exact same way, I thought I'd post this small Instructable about how I fixed and improved them.
Step 1: The Problem
The design flaw is that the handle pivots around a pin that is retained only by a ring moulded into each of the plastic sides. Even though the pin is quite solid, these rings are the weak point.
Stress them too often or too hard, and snap!
Step 2: The Fix
Find a drill bit and a machine bolt roughly the same diameter as the existing pin. The bolt should be a *little* bit longer than the clamp cover is wide. I used a 3/16" x 25mm bolt. (Yes, they mix imperial and metric measurements here, that's what it says on the packet)
Separate the halves of the clamp cover. Using the remnant of the old rings as a guide, drill through each side.
Push/screw the bolt through from one side, replace the handle in the proper orientation, then assemble/screw the halves back together.
Tighten the nut onto the bolt until the handle stops moving freely, then back it off until working well again. You might wish to use loctite or a nyloc nut to prevent the nut from becoming undone.
Step 3: All Done!
These clamps weren't that expensive, but you can never have too many clamps... so it's worth fixing the ones you have. :-)