i had 2 of mine that wouldnt work, one was the back door, and the other was the boot. when i put power to them the wouldnt move.
so i took them off, got a little screw driver on the case and just prised it open a fraction, and sprayed WD-40 into the central locking box, and they manually pulled and pushed the black rod up and down about 20 times. then connected it back up, tried the central locking again and it worked.
i did the same for my boot one.
i havent had a problem with any on the central locking since, that was a year ago.
the power only works a magnet pushing it up and down, they might be stuck like mine, where. or the
have broke , which would cost you.
but try what i did, might save you some money..