"There shouldn't be any "rocks" in your soakaway."
What should be in the soakaway then?
In "normal" terms a soakaway is a hole in the ground filled with rubble
(Rocks if you want) into which the rainwater pipes go.
Sometimes nowadays a hole in the ground is lined with a perforated concrete
ring system, then capped and is hollow but this is not usual, certainly for
properties of more than a few years old