A. The generally accepted time to aerate the lawn is March to November so I am a little surprised the contractors came in winter. Usually the ground would be too hard to successfully aerate if they used a coring machine as it is likely to just bounce across the surface.
The following is an extract from the Royal Horticultural Society on aeration. The better time would be March/April or September/October and if core plugs are removed the grass would usually be top dressed with loam, sand and organic matter to fill the holes left and aid the aeration to the grass roots as well as help drainage. This is a process you would have done every three to four years. If you have a lot of moss then drainage may be a problem and aeration will help but also scarifying carefully with a rake can help remove thatch – the dead grass and moss.
There is an organic moss killer and fertiliser called MO Bacter that kills moss without leaving a black mess or the need to scarify. It is sold in good garden centres or can be obtained from www.djturfcare.co.uk telephone: 01483 200976. It is hard to say whether the contractor is pulling the wool since, as you say, conditions vary across the country, as does type of soil (clay for example can be difficult) but it might be worth talking to neighbours who use the same contractor for their views on why they didn’t have their lawns done at the same time.
Aerating (or spiking) lawns allows better movement of air and water in the root zone. A well-aerated lawn will manage better in periods of drought or waterlogging. For an average lawn, aeration every two to three years should be adequate. Concentrate on areas that receive the most wear and those that are compacted.
Small areas can be spiked with a garden fork, spacing holes 10-15cm (4-6in) apart and deep. On clay or waterlogged soils use a hollow-tine aerator every three to four years. This extracts plugs of soil from the lawn. Hand held and motorized hollow tiners are available. After hollow-tining, sweep up the plugs and then rake a top-dressing (see below) into the holes to improve air and moisture penetration.