Fixed!

A few days ago I fixed the metal zip in Ceri's Timberland boot.

If you can't get the zip pieces to join up and the zipper is stuck, you need to knit the two sides together manually. This is essentially how a zip works by knitting together the little teeth so they tessellate and hold together. Pull the zipper to the top of the zip if possible. Hold the two sides of the zip together and one tooth at a time tap the teeth into place with scissors. Slowly inch the zipper over the bit you have joined so to properly zip it up. Do this all the way down the zip. 

Where the leather sags at the ankles, the zip sags with it and this means when you zip the boot up, the motion is less smooth and requires more jerks. This in turn dislodges zip teeth from their smooth original placement making it harder for the zipper to knit the teeth of each zip side together. Therefore take special care in these areas to move the zip teeth on one side of the zip in line with the spaces on the other side of the zip. Gentle pressure with the closed blades of a medium sized pair of scissors should do the trick.