Any ideas how to improve this uPVC and window combo!?

[Micksup]

As shown in the picture, my property has number of these large window and panel combos. I am guess from the 1980s, they are 3m x 2.5m and all one unit! The insulation and double glazing is poor and the rooms are usually very cold in winter. I could replace the glass with something better but aesthetically they look a bit cheap and I want to improve without removing all and rebuilding. As you will see the window has no cill so need to be careful where the water goes. Have considered building out with battens and then lining and rendering or even cladding and brick slips. But wondering whether anyone had come across something like this before and how they dealt? Many thanks

 

[robinbanks]

Looking to keep the window? Your post seems to be asking for opinions on what to do but doesn't really go into detail as to what you want

 

[Micksup]

This is the front of the house and I need to keep the window and the panel. Inside the house we have radiators attached to the inside of the panel so removing the whole thing would mean relocating that which isn’t something I want (or afford) to do as it would need pulling up significant areas of a parquet floor.
So ideally I would like to keep the window (I will probably replace the glass) but do something with the space below.
Thanks

 

[robinbanks]

I don't think replacing the glass will give you much, insulation-wise, even if you go for triple

In the space below, a rigid insulation infill, foamed in and covered with a decorative rain screen cladding of your choice?

 

[Notch7]

thats 3 windows I think with trims covering the join

take off the trim
put a sheet of celetex across the whole space leaving enough space to fit a cill, tape any joints
screw 50 x 25mm battens vertically over the top
put a bit of timber across the top and screw it in place into the ends of the 50 x 25

then fit a cill across -the dodgy bit is sealing it, as upvc cills usually sit underneath not in front

then cover with cladding -either exterior cement board cladding or maybe a sheet of rendaboard and paint

To do the job properly you should really replace the window -I imagine the bottom part of that window is just external and internally you have timber studwork and plasterboard -so remove the window entirely, leaving the internal in place, then fit new studwork externally, fit new window above.

You might find a new window is not much more money than replacing the glass and it will be a proper job