All I can really usefully tell you is that I love my conservatory and am very glad I had it built.
We used a local firm and I would recommend staying away from the big companies who were hugely more expensive in our case.
And I recommend underfloor heating which is the only real way to ensure the room can be used all year round; we have the sofa bed in ours. Make sure it's gas and runs off your boiler - the electric version is prohibitively expensive.
Cat