General maintenance
If the weather is mild you can lay a new turf or repair hollows and bumps in an exsisting lawn. For the latter, make a ‘H’ shaped cut in the turf, pee back the grass and either fill the hollow with loam, or scraping away the soil from a bump. Re-lay the turf, press it into place and pinch the cut edges together.
Repair lawn edges, especially around flower and shrub beds, with turves cut from other areas of the garden.
If your lawn suffers dieback from treading during the wet, muddy season, then consider laying stepping-stones through it to allow easy access across it without causing damage.
Troubleshooting
Watch your lawn for signs of waterlogging, as the weather gets wetter. If you missed the opportunity to carry out autumn lawn maintenance, then you can still remedy the situation a bit, by spiking the lawn with a garden fork or mechanical aerator. Then fill the holes with a mixture of sharp sand and loam, brushed in using a stiff broom.
Mole activity will increase in January and February due to mating and nest (fortress) building. Remove the largest hills and re-firm before overseeding in spring.
Keep brushing away worm casts, as they can be troublesome at this time of year.
Fusarium patch (snow mould) may be a problem in wet weather, particularly on overfed and lush lawns that have been left a bit too long.
Algae can be a problem on lawns where there is poor drainage, excessive shade, or under the drip-line of trees.