If you're looking at getting new sofas why not consider the storage ones either with drawers or a lift up seat? Your son may just need help with Opening and closing. I don't know how many toys there are but also consider a cabinet in the corner by the fire. I assume the doors are symmetrical in the room although it doesn't look like it in the pic so I'd add your chairs in either corner by the door, as there looks to be room away from the wall, and push the sofas and table further towards the fire. Re light, you could go contemporary or traditional. I'd get something with several arms and hang it about 8ft off the floor. I think wall lights would also be great but then there's the mess with the walls so depends on budget... Do also add table lamps on the cabinet and maybe a side table. It's a lovely room btw

Hello Durnsford - I am not sure of your budget...here's some ideas for custom made storage. If you want a more affordable option then I would suggest Ikea. The first image is from a clients house with a custom made timber cabinet in the living room to incorporate books and storage for toys underneath. The second two bedroom images from Houzz are custom made storage ideas could be transferred into any room.

I think because you have the length/depth of the desk, I would suggest running the lighting behind the desk (so similar to your final example but running in the L shape!). I only have experience using these lights in kitchen bars but attached is a link which would be super easy for you to install yourself. Under Cabinet Lighting

We will be replacing the conservatory as it is cold and not to our taste, so can plan to use all of this space. I am still uncertain if it is worth the cost/loss of garden space to extend all the way across the back. Having the kitchen on the other side does mean not having to mess with the fireplace. Thankyou again for your input.