@Ric, have you looked beneath all the unused coats below the stairs ?
When I got back into my house after 10 years away I couldn't remember alarm code, searched high and low to no avail, decided to fit another box, total pain in the arse, inside looked like spaghetti junction, eventually got it up an running only to find copy of missing code stuck in a cranny beside the box, definitely a ' bugger' moment.
Daughter-in law had same issue as you a couple of weeks ago, it can be reset.
On another note, if your alarm triggers for no obvious reason or alarm does not go off when you open front of elusive box, it'll be the battery, looking at about £11/£12 Toolstation or Screwfix.
Mate of mine had same problem recently and was quoted £100 plus for a service, never heard such, service for a house alarm ! Trip to Toolstation, problem solved.