These batteries fit Fox Carp Fishing Alarms under £40, you need two per alarm. If your alarm takes only 1 battery that looks like this you will need 12V mn21 batteries

The Duracell MN9100 or N type battery is a 1.5V micro alkaline battery, this battery is long lasting and extremely reliable.The Duracell MN9100 can be used in many devices like calculators, automatic garage alarms, small clocks, GPS trackers, auto-door unlock systems, garage door openers, cameras and proximity alarms. This is due to the batteries dependable power and reliability.