Hello, can you help me with my head scratching problem.

I am trying to replace my rear upvc door window with a blank pannel so i can fit a dog flap.. i have come accross glass locks holding my externally beaded glass in and cannot seem to remove them. They are obviously fitted when the door was originally glazed as security, how do i remove them without damaging the glass?... they seem to be right angled metal clips that push into a upvc fitting on the frame around the glass. I guess you need some kind of tool?!? i have no idea of who fitted when as it was all hear when we moved in. i can take pics if ness..