Just a quick post for anybody with a similar problem. We have a Ford Fiesta Style 2007, similar to the photo below:
The electric boot release stopped working recently, and although you can hear the motor whirr nothing happens, so we decided to take a look and find out how it fits together and what was wrong.
Opening the boot you can see the locking mechanism, held in by two substantial hex bolts.
Here you can see the locking mechanism and I've removed the hex bolts (Two either side of the lock mechanism). DO NOT DO THIS! You don't need to for this repair/fix, that unit doesn't have anything interesting in. It contains the mechanical movement, an electric switch (for the alarm and boot lights) which you can see in the top right of the lock mechanism with the red tab (That is a push/pull connector) and two steel cables. These two steel cables are the release mechanism.
To get to these and the motor you need to remove the boot inner cover. This is the plastic cover with the handle to pull the boot shut. It has 3 cross-head screws and then it is pop-clipped shut, so take the screws out and just gently pull around the edges and the whole panel will come off.
Once you've removed the cover you'll see the rear wiper motor, electrical wiring and another black cover (in the middle of the photo, and photo below) which contains the locking mechanism and motor. This comes off with another couple of hex screws and then pops open.
Once you've removed the cover, you will see the motor and locking mechanism:
The two steel cables are then split, one comes into the servo motor shown above for the electrical release. The other feeds to the mechanical switch just up off to the right (out of shot) from the photo above.
As you can see it's a simple mechanism and uses a standard door lock servo to pull on the cable via a metal pivot. The springs them return the servo back to it's extended position.
I found the servo was working, but it wasn't strong enough, so I think the servo is on it's last legs as it didn't have enough fore to exert against the spring. So as a quick/simple fix I decided to reduce the strength of the spring by cable-tying some of the coils together. This isn't exactly a fix, just a bodge job for now!
Hope that helps somebody else wondering why their boot won't pop open, it should be an easy fix once you can identify the spare part to order and will only take about 30 minutes to change.