I have a triangular window in the apex of an end wall in a gardenroom.
I should like to open this window by allowing it to tilt inward [
hinged at the bottom] however as the window is quite large I am
looking for an opener which you see in schools and hospitals
sometimes. It consists of a handle (winding) connected to a spiral
which is lead along the wall and window frame to an opening spiral
attached to the top of the window frame: thus by winding this thing
one opens /closes the window and is secure.

Question: what is the dammed thing called and where do I buy it??

I think what you are looking for is a casement window, however I could
not find any that hinge at the bottem, most of them are small, and
hing on the side:
"AMSCO Windows(R) - Caesments"
http://www.amscowindows.com/notMS/Nav4/casement.htm