Instead of a lattice frame I put up 2x1 timbers across the partition; it made more sense, and of course, it was easier! Started putting the roof in the space. It sounds like it would be a simple operation, however, it turned out to be the proverbial nightmare! I was unable to staple one end of the polythene because I could not get in behind the wall, so it sagged quite badly. So I cut the end section away, and put it in separately. It is not perfect, but, as some colleagues have commented, people are going to be looking at the video and not the ceiling.