406 key reprogramming

i have just bought a 406td. the key i have unlocks all the doors ,but
pressing the button does nothing. the red light flashes on the key so
battery is ok. can anyone tell me how to reprogram it to operate the
central locking. thanks

 

This solution was given to me by somebody in an earlier post, and it worked
for me with almost the same car.

Andy :-

Switch on ignition and press button on key whilst in ignition. keep pressed
for about 10 seconds.

This sometimes resets the key.

 

which MY