There's good and bad news to report.
The good news is that I've proved that I can fabricate a four-sided, sloping conservatory roof.
(And damned tricky it was at times, too)
The bad news is that it has turned out to be far too tall, or steeply pitched if you prefer.
My calculations had failed to account for the overhang and the fact that the main side pieces ended up being a little bigger than I had presumed when I drew it out.
The net result is that like the Indian continent crashing into Asia and forcing the Himalayas ever upwards the ridge line of this conservatory was forced higher than it should be.
The sides should have a much less steep profile.
It's not really realistic to try to reverse engineer what I've got here so the most logical thing to do is to start again and make the pieces smaller this time.
It was only two evenings work wasted anyway.