Thank you for your replies, and glad I asked the question as I've literally no idea in this field so good to get advice from the pro's.
And yes looking at a pitched roof.
I assume the builder would know to get building approval when installing an RSJ, but at least I now know. And I'd never thought about insulation following the work, thinking about I'm not sure if the conservatory has any rads in but will double check as I've not actually brought the house as of yet as it's all still going through. I've attached a copy of the floor plan so you get an idea of the size of the room in the end. As for the windows, we would like to keep the existing if possible
From the replies it's looking more likely that I would need to knock it down and start again.