Spurious Alarm Operations - 12 year old XJ6

[tongu05]

The alarm on my 2008 XJ6 diesel has started to operate for no apparent reason. The first time was about a year ago. The frequency of the operations was firstly every few months, then every few weeks and, now, every few days. It is the vehicle main horn that operates - on, off, on, off - each 'on' bleep lasting for about a second. It can be silenced by pressing the unlock button on the key fob, or, if I do not do this in 30 seconds, it will stop sounding itself after 30 seconds. Obviously my neighbours are now starting to complain!

Any help please.