The following day, 15th October 2015, Rob and I emptied all the sludge out of the pond by hand. We used a bucket and a washing-up bowl to scoop out the knee-high sediment, which we then spread all over the garden among the plants and bushes - back-breaking work. In two and half hours we had just about cleared all the mud out.
Having revealed the liner we were able to make an inspection for puncture holes. We found a few holes, two of them on the pond sides were about an inch long - yes, there had indeed been a slow leak.
As we gazed at huge expanse of exposed liner we knew that the next day's work was already presented; we would have to clean up the old liner quite a bit more.