Most of the early design thinking was carried out using Microsoft PowerPoint and a copy of these outline plans can be downloaded in PDF format here.
But before I had the confidence to make a start on the build, I felt the need to have a 3D computer model, especially of the rafter layout. I quickly found the excellent Google Sketchup 7, which was not only more than adequate for my purposes but also at the right price (free!). Like any 3D modelling software, there was a fairly steep learning curve, but there are a lot of tutorials online and I soon had my required model. For those who have Sketchup installed, this is also available for download as a zipped file here.