Hi,
I have exactly the same issue as yourself which is also explained in the below video (which I think you have seen as well). I am planning to strip the door lock and take a look and I was wondering if you managed to buy a spring. I have found a couple of online sellers on eBay so I might try one of those as they look very very similar to the one in the video.
https://www.youtube.com/watch?v=vIZmJFqoDKk
many thanks,