As you say is not rodable so have help the downspout above the grid level for leaves etc. The 40mm wastes from kitchen however I’ve put below grid level behind. Photo below - probably isn’t textbook but doing the job with no water spilling.
Now...what should I backfill with before I replace paving slabs?
Will need concrete over that pipe because of depth. Should be concrete surround really for protection, also under bends, and gully. Be awkward to do now so if you can at least cover in concrete, clear as much as you can around pipe gully etc: side of pipe first. Well done so far.
yeah great work to date
maybe one of those leaf guards in the running outlet will help
seen so many old systems blocked with leaf mold ,leaf aversion is the name of the game for me now
this is a great start!
LLL