Really nice job Colin. Looks to me as if there is room for some insulation in the ceiling, then you can close off the open end and have a lovely winter hideaway for that G&T. or coffee and whisky as the case may be.
When we first moved here into the high desert, we had the wettest, snowy winter ever, and the house had a cheesy sunroom with plastic windows, aluminum framework and foam filled panels in between. The room was about 3 inches thick foam sandwiched between sheet metal. Hot in the summer, freezing in winter and leaked dust in with every breeze.
So I built a framework inside the outer shell, filled in some of the windows, ripped out a single pane sliding door and put in a real door. A relative put another layer of roof on it, sealing it in with a rubber overlay that he extended well up the roof to stop any chance of leaks. Bought four double pane, aftermarket windows and two sliders. Closed off all the openings without windows, and put a large evaporative cooler through one wall that cools the whole house.
The room became my wife's art studio. Painted a nice yellow, it is bright, cheerful and warm. A few years ago we found the AC/heat ducts had split open, so we had it replaced. We'd installed a slightly oversized HV/AC unit, so we had several 6 inch vents replaced with 8 inch. One of these blows heat into the room during winter so it is comfy as heck in there no matter what the weather.
A couple of years ago, we had a workman come in and put water proofing on the bottom of the outside of the sunroom, then he installed siding and a cap to finish it off. He also redid the soffits, which paint had broken down. Looks wonderful now.
I also have a pretty large patio area not unlike your deck space, but covered. I keep thinking about enclosing it at least with screens around it to reduce bug attacks, but we have a couple of large Bronze color lizards living there that we like. The space also creates a breezeway, nice in summer, but chilly in winter.
We think of this place as our pine box house, because that's the only way we plan to leave it. It is unlikely we will ever get back the money we dropped into it, but it is a real pleasure to live here. Wish I were in better health, Scotland is one place I would really like to visit. I have just enough miles built up to get us both there and they expire on New Years Day.
The more I do, the less I accomplish.