I had this last week as well - would stop going off, lasted about 45 mins. When it finally started to calm down, the little red light would flash constantly........even when the car was started!!
So, I disconnected the battery totally and left it overnight. Connected it back up in the morning, it made a bleep and was fine. Has been since.
I think the bleep meant it has reset itself.
Try this first - save spending £300 notes.
Snagsy