
wow, what a price!!! Their flexiliner looks like what I use as the groundsheet for my tent :) :) that would explain a lot. It doesn't look as though it readily beds in to the contours of the pond, though--as if you need to add water first, which is fine as long as there is no leak or air pocket.
Have a look at LBS Garden Warehouse:
http://www.lbsgardenwarehouse.co.uk/Pond-Liners-DPTPONDLINERS/I have used their butyl liner for years to build ponds and wetland/bog gardens. I put layers of sand and old carpet underneath to stop any root growth puncturing the liner,which is cost effective (you could use layers of Terram if you wanted to spend money, it's cheap) and the butyl liner works well as long as it is protected from light. They offer very good prices compared to garden centres, but that's because they supply so many of them.