If you didn't have the alarm fitted yourself, then you'll have to find out who did. ie. previous owner. Any invoice or certificate concerning the alarm should have been included with the service history and passed on to you when you bought the car. If you didnt get these then write to the previous keeper, who,s name and address are on the registration document, and enquire if they happen to still have the paperwork. In any event if you ring up Toad, they'll tell you if it's Thatcham or not.