Bought one of these (the ConnectModule, not just the manuals). It’s an immobiliser/alarm/GPS tracker. It fits, it works, it makes beeping noises, it knows when my bike’s been moved or stolen, it sends messages to my phone, and I can track its whereabouts. £95 (+ £40 a year after the first year for free) well spent for some peace of mind. Still use a damn great D-lock, too.