It has now developed a leak.
My plan is to recover it with new felt - preferably two layers (or more - as many as I can get out of the role).
I may be missing a trick, but I cannot see any roofing felt on Screwfix's website. All I see are underlay felt. Is that the same thing?
Alternatively, corrugated plastic sheets seem to be pretty cheap, so it got me thinking... What would using those instead involve? I see that I will need screws (as in https://www.screwfix.com/p/vistalux-corrugated-asb-pvc-sheet-fixings-x-200-pack/78225 or similar). But, what would stop the water from running down the underside of the sheet though? Would I need one of these as well? https://www.screwfix.com/p/coroline-corrugated-bituminous-roofing-sheet-eavesfiller-white-840-x-28mm-24-pack/7749j