http://www.flickr.com/photos/23261923@N02/10962763746.
I thought that I would add my experiences to that of Saurus and Sean's. I used the same material as Sean, 3 mtrs in length and 1.5 mtrs wide.I selected burgundy,my original was Jaune Rialto.I used polyurethane glue and allowed 15 cm folds over the bungees for extra strength. I also measured across the roof to get a distance between the raised sections that retain the bungees/roof and the length along those sections front to rear. I also printed off the diagram by Saurus. I would not say it was an easy job but with care and careful measuring and marking out, along with that written by Saurus/Sean,you should get a good result. I wouldn't say mine was perfect but I'm more than satisfied with the result.The hardest part was refitting the rear window,even though I had the correct tool for the beading,it was still a real bitch.
The eagle eyed ones may spot an aluminum section on the corner at the front of the roof,well that is there to stop the bungee hook from popping,as I'd been a bit heavy handed with my plastic mallet,knocking the end fitting in
which broke the edge of the plastic moulding, I've fitted matching plate on the otherside.. All my metal fittings are intact and just cleaned off and painted. I have yet to fit the two side clips and the screw/studs at the rear. I also intend to make a cover to go over the rolled up roof as I just thought it would loo neat. All in the cost was about £75... 
