January, February - Keep the lawn clear of fallen leaves. If there are areas that need re-seeding, provided the ground is not frozen solid, you can prepare these now - dig the area over removing any large debris and weeds, and mix in some compost. Avoid walking on the lawn if it is frozen or waterlogged. Take this opportunity to do a quick lawn mower MOT, making sure the blade is still sharp.
March - If the ground is dry and unfrozen, then it's time for the first cut. Raise the mower blades for a high cut which 'tops' the grass only. If there are bare, compacted patches on the lawn, use a garden fork to spike the lawn to a depth of 10 cm and then brush in a mixture of sand and compost. This will help aerate the soil. Make sure not to mow when the lawn is wet! It won't cut evenly, and fungus can establish in newly mown wet grass.
April - Regularly mow to keep the grass trim and even, but don't cut the grass lower than 2-3 cm from the ground. Take no more than one third off the total length of the grass on each cut. Now is the time to sow the grass seed into the areas you are re-seeding; tread the areas you prepared previously, level them out and then rake to a fine tilth. Sow the grass seed and cover lightly with dry soil, gently firming down the surface. Water in well.
May - Continue mowing to encourage dense growth. Apply a feed and weed product to encourage the new growth and to control moss and weeds. Where you have re-seeded, give the seedlings their first cut when their shoots are 5-7 cm high, but do not apply weedkiller to grass that is less than six months old. You're aiming just to top them at this stage, so raise your mower blades as high as they will go. Make sure the mower blade is sharp, otherwise the young seedlings will get tugged out from the soil.
June, July - Continue mowing. If the weather is hot and dry, mow less frequently and consider raising the blades for a higher cut - less moisture will be lost this way. Subject to hosepipe bans, water the grass thoroughly during dry spells using a sprinkler. Spot treat any stubborn weeds with a weed killer. Scarify the lawn (rake with a spring tine rake) to remove dead thatch and moss. This will prevent brown patches appearing.
August - Continue mowing. Scarify. If there are further areas to re-seed, prepare these areas as in March.
September - Continue mowing, gradually raising the mower blades to leave a longer cut. Scarify. Re-seed areas that require it.
October - If the weather allows, continue mowing. For the final cut of the year you will simply 'top' the grass. 'Top' the young grass seedlings too. Aerate the soil by spiking and composting, just as you did back in March. Apply autumn feed and weed.
November - If you've enjoyed a burst of late summer warmth, one more cut may be necessary. Otherwise just keep the lawn free of fallen leaves.
December - Clear the lawn of any fallen leaves.