UPVC window will not open or close as allen key bolt dropped?

My bottom opening window is now constantly half open as a bolt seems to have dropped on the underneath of the window and I can't open it because the bolt is too low to go over the outside lip but also can't close it as there is a central ridge which again it is too low for.

Looking on my other windows I can see that the bolt needs to be tightened with an allen key but I can't get to the bolt as it is in this central ridge.

Any suggestions welcome!!!

DIY nightmare
March 2011