Over time the grass will blend in pretty well. I wouldn't worry about settlement... I'd just sow grass seed, for the sake of £3 worth of seed you'll have an actual lawn to look at rather than a soil pit with a net over it, and if it ever settles too much for comfort you can rake over a good layer of soil and re-seed at any point in future. Yes, get as much rubble in there as you can, and with 6" of soil the lawn will be fine.
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I wouldn't worry about settlement... I'd just sow grass seed, for the sake of £3 worth of seed you'll have an actual lawn to look at rather than a soil pit with a net over it, and if it ever settles too much for comfort you can rake over a good layer of soil and re-seed at any point in future.
Yes, get as much rubble in there as you can, and with 6" of soil the lawn will be fine.