You have my sympathy!
I have two alarm key fobs that are on their last legs (not been replaced since 2013) and I have no idea how to replace them or remove/replace the alarm system.
I found a Sigma M30 instruction manual for Subaru on the net (
https://www.scoobynet.com). This manual refers to a Multi-functional keypad, mounted within the vehicle, which could be used as an emergency override, and a secure computer controlled data link to the security system control unit, usually mounted under the bonnet. I've not found either of these items on my vehicle. There are other posts on the Scoobynet forum referring to M30 problems and removal.
I have a CoC renewal certificates which state the model fitted is an M30, issued by Raven Security, 8 Bastion Rd. Prestatyn LL19 7ES, maybe they can help you. I don't live in UK, plus I found it difficult to communicate with them by e-mail and phone. So I would be pleased to hear if you manage to find out anything.
Good luck /Paho