Just a note in passing.
I decided to get my plastic windows for the shed from B&Q as they seemed cheaper than double glazing companies.
When you buy them off the shelf at B&Q they come pre-glazed so you have to use plastic clips to fix them into the frame. Double glazing companies usually supply the frames unglazed so that you can fix through the side of the frames with frame fixings then fit the glazing.
I wanted a pair of frames, a left hand opener and a right hand opener. B&Q only stock the right hand opener so I tried to order the other one.
I was told that they had stopped stocking the left hand opener (why??) but that I could order frames from them to be made up. I was also told that this worked out no more expensive because they came with a cill and handles, unlike the stock items.
O.K. Sounded good so I ordered the windows. Picked them up yesterday and they are supplied unglazed with the glass to be fitted. This is an unexpected bonus as the frames are esasier to handle without the glass, and easier to fit with frame fixings than with these plastic clips.
So if you are considering B&Q plastic windows it may pay to explore the special order ones as well as the ones on the shelf.
I haven't enquired if they will make up non-standard sizes but I don't see why not.
HTH
Dave R