Central Locking Works On It's Own!

[wol]

Hi there,
I have a pug 407 diesel.
its got central locking on the key fob and without pressing the button the central locking operates.
I have previously locked myself out of the car and had to buy another key. Ithought that it may be the keyfob sticking but it does this when the key is nowhere neer to the car.
if anybody has any ideas please let me know, also any cost ideas to fix the problem.
thanks paul