Can’t remember the stepoc size but I think it was one less than the largest .
Stone is sat on a separate footing . We had a gap between the stepoc and the lane so just did a concrete pour in the gap . Not very wide - maybe 40cm .
i stuck a land drain around the base outside the stepoc . Covered it in chippings to aid drainage . We had very little gap between dug hole and external insulation. Our dig out was all rock so couldn’t use to backfill