I have finally managed to sort this out! The original document wasn't the right one. There are 2 resources - one for less able students which only requires a pyramid of numbers and the second for more able. The numbers of organisms are included as are the trophic levels. Pondweed may not be on the list, but my pupils recognised this as a producer and hence trophic level 1. Just add graph paper, scissors, glue and calculators.

Not much about on the internet so I have put this together to use as an introduction to the main section of my lesson. It went pretty well, followed up with a plenary of the card sort from another contributor.