The temporary storage shed building and its gutter have been showing signs of being blocked. So we had a look at the down pipe to the soak-away module and cleared out a couple of inches of flowers and leaves! We put a mesh in the top of the downpipe to stop this! But also discovered that the soak-away modules had sunken by an inch or so which means the downpipe is missing the entrance hole! We extended the pipe!
Now it is all sorted!