I have this intermittently with my offside rear passenger door, replacing the actuator looks to be a bit fiddly, but ultimately not too hard. Couple of good videos on it:
Worth changing the handle to latch cable too while you're in there. Apparently they tend to break, and it's a cheap part.
Haven't done the job myself, since mine is just the rear passenger, I've made do with reaching back and manually locking it. Seems to unlock more reliably than it locks.