Andy, You are right the windows are different sizes. The far one, shown in the second picture, was the easiest. All I had to do was enlarge the existing window opening. I put a new piece under the window and pieces to each side.
The other window is taller but not as wide. This meant that the opening had to be made considerably taller but I had about 800mm to the side to fill in. I made a new frame to fit the window and used the boards that I had cut out from the two windows to board the opening, it actually worked out quite well.
Double glazing in the shed is done more for security than any other reason, although the extra draught proofing in the winter will be appreciated. These windows will be a lot harder to break in through than the thin plastic windows supplied. For the same reason, my next job will be the doors. I would like to keep the double doors but have something much stronger and more secure.
BTW, I had a talk to the local "Crime Risk Manager" (yes, that is what the Crime Prevention Officer is called now). He had a number of good ideas for security including fitting a concreted floor anchor and marking tools with a "Smart Water" pen. He sent me 2 booklets and several web links.
Bob