On Wed, 15 Aug 2012 15:15:41 +0100, "gogmagog" wrote:

I was wondering if I
could cover it all up with some corrugated sheets, making sure they slope
down to the back, where there is already a gutter. Is this a reasonable
thing to do, or just plain bonkers?

Bonkers. Probably the most reliable moderately low cost route is to
strip the roof and replace any rotting sheets of material then
fibreglass over it.

http://www.cybglassfibre.co.uk/glass...f-building.asp
http://www.cfsnet.co.uk/acatalog/1project-roofs.html

are two of many suppliers and have instructions on their sites. I
helped a friend do a similar roof and we completed it (including
completely re boarding) in two (long) days.