Alarm problems

[Pistol]

Anyone had this problem before:

When I come to de-activate the alarm it goes off so the only way I could drive it was to unplug the siren and let the internal battery run down.

Everything else seems OK ie the sensors trigger the hazard lights, but at the moment no alarm.

contacted local Main Dealer was told it would need a whole new alarm system at £550 is he taking the mick because looking through past threads I've noticed people have just replaced the black box for about £80. Is this likely to solve my problem?

Its a 98 4.0 Chimaera

[Pistol]

Thanks for all the helps chaps, Its probably worth trying a new siren then after all. Any ideas where I'll get one from.