There is plenty of info on here about central locking - the main issue generally with the symptoms you describe is faulty door lock solenoid (motors). Some suggest replacing the micro switch inside them, but whenever this problem has occurred on mine, I have always got replacements from a scrap yard. My local usually charges a fiver each, and (touch wood) those that I have removed from breakers have all been good.
It's not a difficult job to change them - the door card has to come off, and carefully remove the plastic liner to gain access to it. It's more fiddly than difficult, but straight forward.
In case you are not aware of what it looks like to identify it, it is a black plastic rectangular 'box' tapered at one end with a white hook covered by a rubber boot, about 4"x 1.5" x1".
There will be pictures on here of the offending item.
Earlier models had a different plug, so go for a model of similar age to to your car. The fronts are the same (either n/s or o/s), but rears are different.
Hope that helps.