Citreon C3?

My wifes C3 05 plate 1400 diesel:- Today when she was driving along the horn/alarm started sounding all the time. She stopped and turned the engine off then locked the doors and it was still sounding. When she came home panicking I could hear the noise from inside the house. I could only stop it by disconnecting the battery, but as soon as I reconnected it it started again both with the engine running and stopped. Any help will be greatly appreciated. This is not the 1st time its happened but no one seems to be able to cure it.