Just an update on this:
While my repair has continued to work, I wasn't totally happy that it was a permanent/perfect repair so I continued to search for a more suitable source of flexible 'spring' steel, something that was readily available, inexpensive and preferably required little in the way of modification/fabrication.
Clock suspension springs seemed to be the closest thing available. They come in various sizes, including this one, which seemed perfect:
Ebay link:
http://www.ebay.co.uk/itm/261099848539
Comparing the material thickness, the original material is roughly 3 or 4 times thicker than these, so I figured I would put 3 or 4 of them together, effectively making a kind of leaf-spring, in order to improve the spring strength and better match the rigidity of the original material.
First thing to do is to remove the brass ends. Since the piece of material between the ends is exactly the right length, the easiest way to do this is to snap them off. By holding a brass end in a pair of pliers and bending the steel sharply to 90 degrees and back, it snaps off easily and cleanly:
I found that 4 pieces fitted perfectly and snugly.
I think this may even prove to be an improvement on the original design. Having 4 individual thinner strips of steel should make it more flexible, stronger and more reliable.
Once you know how the armrest comes apart, if you have the clock springs to hand, this is a 10 minute repair (no need to remove the armrests completely, as I did the first time):
- Put both seats all the way back and push the driver's armrest all the way up.
- Pry off both sides of the passenger armrest (these are held with 3 strong clips. You'll need a large flat-blade screwdriver and a bit of force).
- Remove the 4 small black torx screws (2 each side).
- Unplug the phone cradle cable and pry out the plastic grommet it passes through (from underneath).
- Pull the entire insert and mechanism out by pulling it up and forwards from the front.
- Once removed, remove the pull rod from the bottom of it and slide out the black release mechanism.
- Carefully take the release mechanism apart (3 clips).
- Remove and replace the broken spring and re-assemble.