To thicken up your lawn, rake out dead grass, moss and thatch. Top dress with Lawn Dressing which will result in denser grass growth and level out minor hollows.
If you have bare patches in the lawn that need re-seeding, now is a good time to do it. Gently loosen the soil with a hand fork and remove any weeds. Then water the area if it feels dry and level out. Scatter the grass seed evenly over the area and cover with a thin layer of compost. Netting can also be put over the area to help deter birds.
Once the grass has reached a height of 5-8cm cut it on a high setting for the first time. Keep the grass lightly trimmed and gradually lower the cutting height until you are happy with the desired effect.