It's heavy canvas - have dismantled and reassembled. Nothing obviously wrong - have tried adding sikaflex. Maybe waterproofing treatment for the canvas?!
There was an article in PBO many moons ago about replacing a mast boot with some "new" type of boot. I have no idea what the PBO number was nor exactly how long ago! Maybe it is possible to search (via the net) the magazine articles for the appropriate one?
Alan.
Edit: Just checked and there is a search function for articles. Found this one which may be of interest to you:
Yacht Boot - neoprene lined rubber boot
Mar 2008 p109 (0.10 pages)
View Related Articles
Category: Gear & equipment
Author: Rob Melotti
Description: Chandlery
Yacht Boot - neoprene lined rubber boot Buy Now: £1.95
PDF Download available now only £1.95
Mine is made out of the hypalon material that inflatable boats are made out of, and seems to work, leaving only the water running down the cable runs in the mast and the condensation from the inside of the mast boot to deliver water inside the boat! As previous poster AngusMcDoon alludes to, I will regard deck stepped masts with a renewed appreciation.