The description below may sound a bit confusing. That is because I found it difficult to put into words. When you are doing this it will be obvious ( I hope ).
The Door Locks were easy to sort, once I had worked out how the complete Door Mechanism worked. The Door Handles are held on by two screws. One is accessible next to the Door Catch ( The bit that locates on the Striker Plate ) and the other screw is accessible by removing the Door Panel ( Some call it the Door Card ) and making sure that the Window is in the fully up position. There is a lever coming out downwards out of the Handle assembly that is attached to a rod. This rod just pushes off as it is held by a small clip.
Once you have removed the two screws and the rod the handle will come away from the Door, it may need a bit of a twist to get the lever out with the Handle. Once removed take off the round spring clip and small screw that holds the plate on and remove the plate. Note the position of the lever as this is important in the operation of the locking of the Door. Take care as there is a spring that will push at the lever. You will now be able to remove the Lock assembly.
You may need to put the Key into the Lock. Once removed remove the Key and pull out the small brass sliding plates and small springs. The springs can be quite tight. Take note of the position of each of the brass plates as they are positioned in an order relating to the Key profile. Clean the springs and brass plates and clean the groves that they fit in. If you use a Junior Hacksaw Blade, snap the end off the blade and file the blade so it is half its width it will be a perfect size to clean the groves in the Lock Barrel. Once all cleaned give a light covering of grease and reassemble making sure that the brass plates are in their correct position to suit the key profile. If you stretch the springs slightly before refitting they will regain their tension and will push the brass plates into the handle groove better. Fit the Key and make sure that the Key turns OK. The way the Lock works is when the Key is fitted the brass plates move to the center of the Lock Barrel. When the Key is removed the brass plates are pushed outwards by the springs and the brass plates engage in the grove in the Lock barrel. Clean the Lever and other components apply a light smear of grease and reassemble. If all correct when you fit the Key and turn the lever should slide. You will note that when the lever slides there is a slot in the lever. This slot has a narrow part that moves to prevent the push button from being pressed in. When the Key is moved in the other direction the plate slides for the wider part of the slot to allow the push button to to be depressed. On the threaded rod that is attached to the Handle Push Button there is a nylon sleeve. This sleeve. needs to be in a position that allows the lever to slide between the sleeve. and the sleeve. closest to the Push Button. It is difficult to explain but will be obvious when you look at it. Refit the Lock to the Door and refit the small Rod to the bottom of the Lever. The way everything works is for the lever to prevent the button to be depressed and when the interior handle pushed to the locking position the rod across the inside of the Door pushes on a lever that pivots and moves the rod that is attached to the lever from the Door Handle. So, when the door is locked if the interior handle is moved to the unlocked position it moves the rod across the inside of the door, pivots the lever and moves the lever attached to the Door Handle to the unlocked position, that is it moves the Door Handle lever so that the slot in it moves to the wide position and so allows the Push Button to be depressed. If the Door is unlocked and the interior handle is moved to the locked position it slides the Door Handle Lever so that the slot moves to its narrow position and so prevents the Push Button being depressed. When testing to prevent you having to keep closing and opening the Door put a screwdriver into the Door Catch (The part that locates on the striker plate) and manually engage. Make sure that there are two clicks as you push with the screwdriver. By doing it this way it will ensure if you have done something wrong you will not jam the Door in the closed position.