There are a couple central locking fixes around, but I found this one here:
http://www.mivec.co.uk/q_and_a/q_and_a.htm
Havnt tried this one yet myself. But I think ill have a go at it over the christmas break.
Q. My central locking is a bit sticky - what can I do?
A. This is a solution, courtesy of Peter Sarsota
Removing the door panel.
First remove three black plastic popper covers and the screws behind them. Two on the rear edge adjacent to the door mechanism and one at the front in the panel dashboard recess. Remove screw just below inner door handle and then slide the plastic cover that it held on forward whilst pulling away from door. Remove the large screw that is now exposed. Remove the plastic plug just above the door pocket and remove the screw behind it. Carefully lever the panel away from the inner door skin using a screwdriver at the bottom and then pull the panel off the door. Disconnect the plugs for windows and speakers. Carefully pull the plastic off the door and remove the one inch wide black plastic disc adjacent to the locking mechanism just above the rubber door stop
Removing the lock mechanism
An inspection lamp is really useful now! Look inside the door and you will see where the rods for the internal handle and lock joint the locking mechanism. They are held in place with green (may be orange) plastic clips. Easiest way to remove them is to follow the rods with your hand towards the lock until you feel the plastic clips, push the clips up off the rod then push the rod towards the outer door skin. Next you need to disconnect the two rods for the outer Handle and lock. These disconnect at the handle and lock end (You cannot disconnect them at the Locking mechanism end) The lock rod (shorter of the two) is held by a similar plastic clip. Again follow the rod up until you feel the clip, push the clip off the rod and push the rod back.Now the really fiddle one. Unless you have very small hands and rubber arms you will not be able to get a grip of the top of the last rod. Pull the rubber out above the lock on the end of the door where the door and window meet. Look down though the hole you should see the rod and clip, use a screwdriver to prise the clip open and remember the plastic disc you took off, there was a hole behind it. Put your finger in the hole and push the rod out of the clip. Undo the three screws around the door lock, hold onto the lock or it will drop to the bottom of the door. Manoeuvre the lock mechanism behind the window guide and out of the door. Disconnect the cable plug.
Fix the locking motor
Remove the 6 little screws that hold the circular cover onto the locking mechanism. Don't lose the rubber seal.
Inside you will see cogs, a little motor and a green disc shaped electrical component. The problem generally seems to caused by the grease in the unit melting and getting into the motor this in turn causes the disc shaped component to overheat and break down reducing the power to the motor stopping it turning fast enough. Easiest way to check is to lift the motor up at the top so the spigot does not engage with the cog and plug the unit back into the car, Every press of the remote button (note you will have to hold the interior light switch in or it will not try and lock/unlock) will result in the motor turning slower and slower every time you press and the disc shaped unit geting very hot !
DO NOT TEST WITH THE UNIT OPEN AND THE MOTOR ENGAGED OR ALL THE BITS WILL COME FLYING OUT !! YOU HAVE BEEN WARNED.
Two ways to resolve this.
1) Very fiddly. Un-solder the two connections to the motor (Make sure you mark which way round they go) Place the motor in a vice spindle down and lever the two metal tabs off the plastic end plate. The motor will now come apart. Gently clean the brushes and conductor with a degreaser. Use two opened paper clips to hold the brushes out the way while you put it back together and tap the metal tabs back into place. Re-solder the two connections.
2) May not resolve the problem. Un-solder the two connections to the motor (Make sure you mark which way round they go) and then spray electrical board cleaner (Not WD40) into the motor several times. Wait until cleaner has evaporated and then reconnect motor.
Test again by connecting to the car. Motor should now turn quickly. If not then electrical componect is probably knacked. If this is the case just bypass the electrical component using a very short piece of wire. Needs to be short or you will not get the cover back on. I just soldered a piece of wire from one leg of the electrical componect to the other. Test again by connecting to the car. Motor should now turn quickly. Reduce the amount of grease in the unit especially above the motor (to stop it happening again) Put it all back together making sure the cover fits correctly. (It should fit flush all the way round without pressure. If not then the cog spindles are not located in their holes correctly.) Very important or lock will not work properly !!
Replacement is the reversal of removal. Except that you when you reconnect the outside door handle rod, do not lift the handle up. Check carefully that the rod is correctly engadged in its clip. Careful the clip is easy to break.I am unsure why the electrical component is there. General concensus seems to be that it is to protect the motor from an overload or to reduce noise picked up on the radio. So it may be that the motor will fail earlier than normal. It is up to you. Take the risk or lock your doors manually !
Peter@sarsota.force9.co.uk