Yeah mate, exactly same problem I have. Same door too. It is the locking mechanism. Is it an 03-04 by any chance? The locking mechanisms on these earlier ones seem to break often.
Chris Rogers, from this forum, will be fitting a new mechanism for me when he comes down to Melbourne in August, it costs a little over $300 bucks, plus installation.
At the moment I do the same as you, I either lock the car manually before I get out by moving the latch when the door is open and I'm getting out, or lock it with the key.
Unfortunately everytime you unlock it to get back in the car, you will have to use the key for your door.
But yes, you're up for a few hundred to solve the problem.