Out of interest if I order a new card will it (and my spare) be programmed so that the old card should it ever be found stop working or will they all (new and lost) share the same code.
I remember reading somewhere that new codes will be programmed so the old one would not work anymore.
Based on VelSatisfied's situation I suspect not!
As I understand it, the coding works two ways:
1) The card is programmed to match the serial number in the car's various computers.
2) The car's ECU is programmed to recognise the unique and separate serial number in the card.
The first point is the reason why second hand cards are useless. The cards can only ever be programmed with one computer system's serial number - it's a one-shot programming system.
The second point can effectively be used to disable lost/stolen cards. The car will only recognise the cards that were present at the most recent programming. Any cards that were not taught to the car on that occasion will no longer be recognised, even if the car had been programmed to accept them in the past.
So your lost or stolen card should be doubly useless now. It can't be programmed for use on another car, and your car will be programmed not to recognise it when it's paired up with your new and existing keycards.