So a proper lintel was bought (they are very cheep!) to make it look like i'd meant it to be that way. I put great big bits off wood on top. They are called i'm told "wall plates" and are more usually laid to one side of the wall if you are at all compitant, but i was as ever making it up as i was going along, so i did it and odd way and had them on top.
Gawd it hard work for a lazy person. Here is the roof sort of on, well the beams in (that's a hundred quids worth of wood!) I then put "noggins" (short bits in between the long bits) in which firmed it up to a surprising extent. Then i nailed most of four sheets of Mason board over it. (almost another hundred quid!) I made up frames to match the windows i had... well in theory anyway they were exactly the size of the windows.