Bodge/Pete, I did it the simplest way possible.
I made up a 'batt' of bubble wrap (several layers deep) cut to fit inside the panels formed by my roof's timbers. As I'm a cheapskate, I had used 3"x2" timber, so I think that meant 3 layers of wrap, held loosely together with any old sticky tape..
I then stapled a cord on one side of the first roofing bay, at roughly centre-line, pushed the batt up into place, holding it with one hand, then stretching the cord across and quickly stapling it into place on the opposite side. That meant I had the batt draped over the cord, but it was at least in place. 15 seconds and 2 more support strings and the batt was filling the bay, sagging a bit at the corners.
The final touch (and it's a good idea to prevent interstitial condensation) was a scrap piece of clear DPM, maybe 1200 guage, from a building job (you could use any clear plastic) placed over the batt's lower surface and edge-stapled all the way around the 'bay' at about 2-3" centres onto the roof timbers.
I found I didn't need to Tyvac the edges or anything similar. The DPM makes the the job look tidy, even where the edges may be rough cut or rough stapled. Four years on, no problems.
Sam