Can anyone help identify the aquatic larva in the attached photographs?  We found it in a pond as part of a mini-bioblitz at Queen's Road Allotments in Leicester.  It's movement appeared very odd, as the legs seem set a long way back.  I thought it must be a snakefly larva, but they are terrestrial.  It doesn't look like an alderfly larva.  I have tried searching using terms like megaloptera and raphidioptera without finding anything like it.The tadpole and mayfly larva in the photographs give scale.  It was probably 2.5cm-3cm long.