thank you so much for all your answers. I only get back now as, since my initial posting, I tried to put my dahon in the hands of two shops in order find the parts or solve the issue somehow, but with no success. Now in holidays I get back at solving this problem.
So thank you for your comprehensive solution smallwheeler, but I've found another problem. Yesterday I removed the handlepost, removed the quil stem to find that it its diameter is 24.4mm. I can't seem to find any place where I can get this part....
Furthermore, I have sent this to solder, but it seems there was a problem with the alloy or with quality of the actual soldering work as it broke off easily.
So now I'm wondering what to do next... Any sugestions? Many thanks