We've done somthing similar Beccles - the initial idea was a conservatory which turned into an extension.
If you are having all of your back wall removed I would go with a builder because that's a lot of house to hold up and you'll need new support putting in. Have you had some plans done? Ours cost 20k with the kitchen and a downstairs loo but isn't as big as yours sounds. You mention a hard roof do you mean tiled, are you having a ceiling or having it vaulted?
Were having the same thing done now. Our house has a seperate kitchen and next to that a dining room. We are extending out along the back of our house so our kitchen will be rectangle shaped and off that to the side a seating area which is connected by double doors to the dining room. Originally I wanted the kitchen half to be half wall to cupboard height and then half windows with the seating area conservatory type windows both with a glass roof but the archietect who came said a kitchen in a conservatory wont work due to steam and condensation.
So were going for the same shape but with a sloped tiled roof with velux windows in and then a normal sized kitchen window with french doors and a window in the seating part.
The kitchen will measure 3.5m by 6m and the seating area will be 3.5 by 3.5.
Weve been quoted 18k for the building, plastering, electrics and went to MFI who quoted 3.5k for kitchen units.
Weve gone for a builder due to the tiled roof and building work needed.
hope this helps.