Hi @CharlieCT.
Usually, the key can be reprogrammed back in using the little keypad fitted in the "coinbox" cover by your right knee on the bottom right side of the dash. No need for a new key.
Do you have:
1. The little manual for the UK / Sigma specific alarm system, and
2. The code number for the system in your car ( 4 digits).

If not, it can be reset using a generic code. Will dig it out of my manual and post up tomorrow ( unless the genius on the forum @don1952 does it first!)

 

Do you know your PIN for the alarm keypad?? Do you have the alarm user manual?
if not..
If you have a working key the first thing to do is change the code to one you know.

Changing your PIN:

• Arm and then Disarm the system using a working Radio Key.
• Within 30 seconds of disarming, type into the keypad: *17856*92*wxyz*wxyz# (where wxyz is your new PIN code)
• Arm the system with the Radio Key
• You should now be able to disarm the system using the new PIN code.

Having changed the pin the next step is to re-pair both keys to the alarm

The link below has this procedure and a link near the bottom of the page has a link to download the user manual

Scoobypedia | Trusted knowledge for everything Subaru | Knowledge / Reprogramming Sigma M30 Alarm Remote Controls and Key Fobs

 

it's also possible that the switch inside the key has become detached from the PCB.
has it worked at all since you've owned it?

 

Hi,
Have a look here:
Key fob suddenly stopped working
I had a similar issue, instructions to repair the key are included in that thread (post #9)
Good luck!