Central locking problem on Golf

Hi

I have a problem with my central locking on my Golf - 52 plate, 1.9 GT TDi.

I can open the driver's door by pressing the key once. But when I press the key twice, it doesn't open the remaining doors. All doors close using the key.

Central locking was working fine until I gave the car to a mechanic to replace a part. It hasn't worked since. Mechanic denied having done anything that could have caused this to happen.

I haven't done anything about this so far as doors open/close using the button on driver's door. But it is a pain though.

Any advice would be much appreciated.
Thanks