Sow seed over the existing grass, around 25g per square metre. Rake gently, then roll the lawn. Apply an organic fertiliser and water in well. When the grass is 5cm (2in) high, cut again.
The perfect time to lay a new lawn from scratch is early autumn, when the grass can root easily ... When turf arrives, stack rolls in a shady spot and if they appear dry, sprinkle with water.