We were so frustrated with this same dilemma several months ago, and ended up finding Rock on a Roll. We didn't even have to empty the water in our pond or waterfall, but just moved rocks that were on our black pond liner, and laid down this rock-like liner over it. It feels like rock, it looks like rock, but it is super malleable. What is nice is that it is really thick and strong so it doesn't puncture, and was thus easy to work with. Just had to share since it saved us a lot of headache, and was fairly cheap. $30-40 for 3ft x 8ft roll. If you want to see before and after photos I wrote a post about it on my little blog to help fellow pond owners hear about it:
Last edited: 11 August 2017 21:12:44See original post
Sadly we cannot get it in the UK.