Astra G 1.8 SRi Alarm Problem URGENT!

[aruck]

Help!!
The alarm on my 2001 Astra keeps going of randomly.
It did it a few times last summer and then nothing until this week. I'm assuming for some reason it is heat/sun related?!
I turned off the internal movement sensors last night using the 2 internal light switches as the car manual but the alarm has gone off 5 times today.
Do I need to turn of the sensors every time or are they off permanent now until I hold the switches in again?
What else could it be?
Can I turn alarm off?

Thanks