Central locking discovery one not working

Hi people,
My central locking has just stopped working with the key. If I sit inside and depress the central locking button on the drivers door everything locks. When I try to use the key it doesn't feel much resistance, hence no locking. Could it be a simple matter of adjusting/tightening a cable or rod? How or what should I look for and is it a difficult piece of work?
Thanks very much

would start by taking the door card off and seeing if anything has come away / lose.. have come across central locking failing from the key, usually caused by electricals but if the drivers door isn't locking either then it sounds a bit more like something mechanical