Hi again, and thank you very much for the advice, I appreciate it a lot, since I haven't managed to take any pen apart before.
The link to the PDF file is very helpful, my only problem now is taking the pen apart.
It appears to have "grown" together at the "seams", I tried using a little gentle heat - that didn't work. Hmmm.
But at least the cork seal inside is so dry that I could get some clean water past it without problems. And I can shake the water out of the button end without problems, so at least I can get the pen cleaned inside.
As I write, the pen is soaking in clean water in a bowl in the kitchen.
Thanks again, the pdf file and your advice is wonderful help.