Cheers, Hugh. I hope so, I've found the forums very useful over the years so it'd be nice if some help could go back the other way for a change!
Onto the next stage, finishing the sloped ceiling panels. I wanted the edge of the panel to sit completely flush with the ceiling rather than be bordered by a timber frame, and the angle of the pitch roof made the cuts rather interesting. I started out by marking out where the panel should sit, and taking some measurements along where I wanted the braces to sit. I like just cracking on with a saw as much as the next person, but some planning and paper definitely helped here.
Marked out:

The simple half of the frame:

I then clamped that half in place, and double checked measurements for the braces.
Angles on the edge of the braces:

Because of the odd angles, I cut the braces slightly longer than necessary and then trimmed down the straight end once the angles were fitting nicely. This allowed a bit of room for error, and cutting off the other end at 90 degrees is a far simpler task than trying to shave a few mm off of the angle you've just got to sit nicely...
Another test fit with the braces attached, the longer one does fit, but gravity is doing its bit. Honest:

You can never have too many clamps:

A couple of shots with the rockwool in:


Final test fit:

These came back down as they were getting fixed together and placed up as one panel once I'd got the fabric on. I also got another few slabs of rockwool behind them before fixing them in, so at the deepest end theres around 250mm.