Drivers side door central locking faulty. Help please.

[X5_twin]

My drivers door does not unlock using the key fob. All other doors unlock and lock fine with it. Drivers door can only be unlocked using the key or using the button on the dashboard from inside the car.

Any ideas or checks I can do to determine exactly what the faulty part is? Don't really want to pay the garage to spend an hour "investigating" the problem! Also any idea on costs of parts and labour to fix the problem would be great?

Thanks in advance for any advice or help.