i did it in a day on my last td5, bout £75 give or take, not that hard at all oh and i could remotely start it with a button hawk car alarms, got it off ebay
i believe you can get a kit off ebay that has the actuators that attach to the lock mechanism.. then connect a remote alarm and you have remote central locking!
there is a thread on here where some one did it.. il dig it out in a mo.
I bought the alarm off Ebay including central locking motors, total cost £37.50. It took about 4 hours to fit. My Landy didnt have central locking to start with. Poor picture but you can see how it goes. Also fitted electric windows at the same time they cost £45 and took 2.5 hours to fit.
the kit i got included the motors and all wiring etc. it couldnt have been easier to fit it either, and as for the motors being weak, they were more than capable, but make sure you pick a proper solid place to bolt it to so its not rattling around in your door