Resetting key code for spare key
 

I picked up my new (to me) Ford Ka at the weekend and it has 2 keys.

Both keys work on the doors, but only one will start the engine. I read in the manual about resetting and re-coding the keys and it seems to work fine for the second key. If I code it, and use the second key, it all works fine. However, if I try after an extended period, it doesn't work any more.

What am I doing wrong, and how do I re-code the spare key permanently?

Thanks