Step 3: Modifying the torch head
The head has, as expected, most of the important components within - there's an exploded view below.
My approach was to dispose of the bulb, and use the soldering iron to melt down the reflector to accommodate the circuit board. I also trimmed the square edges off the board with the wire cutters to help it fit a little better.
The ideal would be for the LED module to fit into the reflector assembly so that the tops of the LEDs lie flush with the front of the reflector - this would mean it can be refitted into the lens assembly without any further modification.
In the event, though, I had to opt for hacking at the reflector to retain the thread for the bottom cap, and use blu-tak to rest the module within it. I found the easiest way to 'cut' the reflector to size was to use the hot soldering iron bit to melt it, trimming off the excess with a craft knife. Ensure you have good ventilation if you choose to do it this way, since it can emit some nasty fumes.
Now's the time to solder the loose wire to the metal negative circular plate. If the reflector assembly is still intact, this will require another hole to be made in it, to allow the negative cable to reach the plate without going near the positive terminal.
Next trim the 'positive leg' so that it comfortably rests on the springy contact of the positive terminal in the screw-on cap. To facilitate this, I bent the terminal up a little so that it could get as good an electrical contact as possible.