Thanks for all your helpful thoughts. My intention was to make life as easy as possible for this project, but the nature of the building I had to work with has made it anything but. The room I have to work with is an old Scottish granite Steading (barn). Being made from granite blocks, everything is uneven. Anyhow, the stud walls are all nice and squared off a treat, but the problem comes when it comes to the apex roof. In my wisdom I thought I would leave exposed the wooden beams, but this means I have three 'panels' of roofing plaster about 1mtr wide on each side of my roof running at about 45 deg. Hard enough for a novice, but add to that is that each of the sections is completely out. So I've managed to get about a third of the plastering done so far, and fun isn't the word I would use. Ive tapered edged as much as possible, but what with working on my own with 12.5mm board over my head and at heights, it's a minor miracle there aren't more gaps I need to fill, let alone a mixture of butt and tapered joins!
One good thing though, it's rustic. Flawless plaster just wouldn't be quite right. That's what I keep telling myself anyway.
I really should upload a pic if anyone's interested. It's certainly been a challenge.