You just need to plan ahead. I started with the back which was against the fence (about 6 inches away). From inside the frame it was possible to lift the glass through the centre and carefully move it into position. The clips can be inserted by reaching through from either side. I continued with the middle section. Here I was a bit harder to get the clips onto the far side but my arms are long enough to reach through (still inside the frame) and across the 2 feet wide panel. The final panels on the back could be reached from the outside. Next I glazed the side near the shed with a similar procedure. To glaze the roof the panels can be lifted through from inside and into position. I had a step ladder inside the frame so I could stand up an insert the clips from above.

I managed the whole job by myself, but if you have an extra pair of hands to help you it would be a lot easier, as they can hold the glass in place whilst you get the clips in.

The main drawback of this approach is that it will be very difficult to replace a broken pane. If one ever breaks at the back I'd have to remove other panels to get to the broken one. It's in a very sheltered position so the main risk will be if I break it from inside!