Rover 25 alarm keeps going off

My car alarm keeps randomly going off. I use the fob most of the time but I have used the key and it still happens. It's been happening on and off for the last three years!!! Sometimes it cannot go off for months and other times 3 to 4 times in a day. It happens throughout the year. Not even particularly in wet/windy/cold weather. It happens at work, home and carparks and can go off anytime during the day or night. Sometimes I push the fob so that it unlocks the car and turns the alarm off but sometimes it stops the alarm, I think the car is unlocked and the alarm will go off again and I then realise that the car is still locked!! I have to leave my car unlocked at work because it goes off so often. Please help as I think my neighbours are getting just as fed up as I am.