Thought I'd add my input as I've just replaced a faulty lock on my Ka and found andy's post very helpful in doing this. Biggest problems I had were:
1. figuring out how to remove the window winder handle - use a small screwdriver to bend out the plastic sleeve and there is a metal clip, pry this loose and the winder pulls off.
2. the anti theft cover, as andy states this is rivetted in, the rivets need completely drilling out. Afterwards you will need to secure the cover using nuts and bolts.
3. Removing the retaining ring - mine was slightly rusty and I had to use some plus gas on it, then using a screwdriver in the closest hole around the outside edge of the ring (there are two) use a hammer a knock anti clockwise until free.
A fiddly job but worth it when done.
Moppet
February 2010