I had same issue on my MKV, I did spray some WD40 on lock, and it seems to fix it. Remove little rubber cover with lock picture located on the side of door, and spray WD40. 2 month now had no issues so far.
Have you seen this post?http://www.mk5golfgti.co.uk/forum/index.php/topic,48207.0/topicseen.html
Have a look at my posts in this thread. I has exactly the same thing. http://www.mk5golfgti.co.uk/forum/index.php/topic,48079.msg554184.html#msg554184