One of the two keys for the 406 (we carry one each) developed a problem when the large button (the central locking one) fell into pieces leaving an open hole with the microswitch visible but difficult to operate. Took the key apart and removed the circuit board (it is clipped into place). Then removed the remains of the button, which fits like a grommet, this leaves a larger hole. Then I cut a piece of old car inner tube to fit over the hole on the inside, making sure that it did not extend far enough to obstruct the second (deadlock) button. This piece was then glued into place and the key is back in service and works fine.
Hope this helps anyone else with similar problems, it is much cheaper than a new key.