Spring and Autumn are the best time to sow a new lawn or rejuvenate an old lawn.
Lawns establish better from March to mid April when there is still heat in the sun and autumn rain fall will give a good strike.
Preparation is everything...
First remove all perennial weeds, such as couch and dock, with a fork and then with a rotary hoe cultivate a seed bed about 20cm deep. Level out any hollows. Use a long board as well as much tramping with your heals - this will find all the soft spots that would sink and create hollows after the very first rain. A roller is not suitable at this stage for developing a level surface as it will simply roll the mounds and straddle the low bits.
If you are wishing to have a "show lawn" sow with fine leafed grass seed such as Fescue and Browntop.
For a hard wearing lawn that suits family, pets and sports select a lawn mixture that contains some Rye in the mix.
Because the seed is so small divide the whole lot into four parts and sow up and down the area then across as well as diagonally for an even coverage. Rake the seed in very lightly and roll. Moisten each morning to stop the birds from enjoying wonderful dust baths and you will find cats are not fond of digging damp soil either. Moisture is also vital for germination. When the grass is 6cm to 8cm high it may be cut with a lawn mower set high.
To give old lawns a life - Spray out dandelions and daisies and rake or scarify up old thatched grasses that tend to choke out a lot of lawn grass which is often the cause of a thin patchy lawn. Fill in the low areas with soil and tramp, rake, level and sow lawn seed. Scatter some seed over scarified area and sprinkle a little top soil on top of the seed and keep it moist to help the germination.
If you detect Grass Grub, get onto removing these immediately as it can destroy your lawn.
Best option is to mow your lawn first then sprinkle Neem Granules over your lawn at a rate of 50gm per sq metre and water to settle into the soil. Ensure the areas where damage has occurred are especially treated as the adult females lay their eggs back where they emerge. Best times for treating is around December as the females are laying their eggs and the grubs are hatching. Next best time is autumn when the ground is softening up with the autumn rains.