In the summer cracks can appear in your lawn, they can be very unsightly. The cracks can be filled with a fifty-fifty mixture of sand and soil less compost or peat. You can re-seed at the same time. It should not be necessary to repeat the exercise more than a couple of times
Bare patches in your lawn can be removed by using spare bits of turf (cut from any new flower-beds, etc.) cut to size and fit into the gap. Add plenty of compost into the cracks, and water well.