I found a tear in my bag's rain cover the other day. After looking on the web for a suitable patch to fix it and seeing some quite absurd prices with even more absurd postage costs, I thought I'd try a Park Patch as I've got a load sitting about.
It's gone on beautifully to be fair! A few days in it's still holding a perfect seal too. Will see how long it lasts and keep an eye on it, but so far it looks like a neat and cheap solution to a small tear in waterproof fabric