Hi Morton, thanks for your reply... I found out why it doesn't work. The focuser extends to a maximum of 47mm. The eyepiece needs about an additional 3-5mm. The 35mm extension tube brings it (focuser back in) to 50mm. Unfortunately the eyepiece doesn't slide completely in the extension tube (it stops where the thread for the filters starts), so it sticks out even further... to far. If the eyepiece would slide in completely in the extension tube, it probably would work...
Thanks a lot for the link, I'm pretty sure this will solve the problem...

. Now I'm sitting on a extension tube I don't need... oh dear.
I really appreciate your help.
Cheers
Tom