This is the first conservatory we've had , only been in the house for 7 weeks but at the moment we love it got cane type table & chairs & cane sofa & chairs eat meals out there listen to music etc. Guy at work was quoted £6000 to have a "proper" roof put on his declined the offer.
Cheers for that. I might get a quote just for a laugh. We do use it in the winter, particularly with dinner guests, but I'm just so mean I hate turning the heating on. Mind you, I get free electricity from my solar panels, so the underfloor costs very little, and a fan heater warms it very quickly.
Get a radiator fitted that’s on main house feed, that’s what I did. Helps a lot.
See #7. Two built in from main CH.
Sounds like it's all there, if you want an insulated roof it really does sound like making it a brick extension is best. You could re-use the double glazing windows in some parts, brick elsewhere so it supports a standard, insulated and tiled roof. I did strongly look at what Fuse19 mentioned, baton structure with insulation pushed in, then boarded and plastered. It's a hack, it's not right and only way to do it internally.
Other way is what I've seen top section is replaced or added to, but I doubt the conservatory itself could take the weight. Then there is the cost, given size of yours, it would be £10k easily to do that.
My conservatory does not have decent foundations, hence I'm starting from scratch.
How far does the conservatory go out? You could get it done under permitted development, as current footprint of existing extension.