Central locking/alarm problem

[paulplymouth]

My Boxster S 3.2 (Spanish import) is 2 years old and has done 4500 miles (summer use only).
It has 2 identical ignition/locking keys which worked fine until recently.
On one key the internal battery went rather flat so the little battery in it was replaced.
The replacement was done after the car had been centrally locked using the key fob button. With the new battery in the key it would not then unlock the car via the key fob.
Since that time the key fob will not work the central locking properly.
The red light in the key comes on when the fob button is pressed but it will not set or unset the central locking or open the boot/bonnet.
Worse than that: the key can still be used in the door to centrally lock the car but after unlocking in that way and opening the door the alarm goes off until the engine is started!

The other key still works everything fine.

The change of key battery seems to have prevented the key fob button from communicating with the car's locking system but why and what to do. Any advice appreciated.

[Britwrench]

Needs to go to a dealer or specialist with the PST2 or PST3 tester for diagnosis.

At two years should still be under factory warranty so go to OPC.