Monday morning came (this morning), Matt turned up, and one of the first things he asked after having a look was "do your hands hurt?". Essentially, he figured if we had been doing the right thing our hands should hurt. He showed me what to do, and after about 10 minutes of thumping downwards on the outside of the screen (standing on my painting trestle), it slowly edged in. A bit more thumping and the top rubber lip went over by itself.
I took this picture Sunday before it was all the way in.
(Later addition to this post) This is the "trick" in getting these windscreens all the way in.
Once the bottom and sides are in, if the top still has a gap do as indicated in the picture. As Matt said, hands start to "smart" after a minute or two.