Do those circles correspond with the position of the conifers you had removed? Did you have the roots taken out?

No the conifers were in the hedge boundary to the right of the lawn. It has been suggested it might be to do with the change in light levels?

If there's grubs in the lawn (leatherjackets or chafer) you usually see birds pulling at the grass, often yanking it out in clumps, to get at the grubs beneath. You can check if its leatherjackets by leaving a sheet of plastic over them overnight, then lift it early in the morning - if there's grubs, they should now be on the top.
There are many causes of brown or dead patches in grass, from simple things like over application of fertiliser/lawn treatments, poor lawn maintenance, canine urine, along with other infestations and infections such as red thread or fusarium patch. Diagnosis is down to close examination of the grass, what time of year the patches appeared, what kind of grass you have (standard rye mix or luxury turf, for instance), what kind of soil and whether the weather's been very wet or very dry.
You could get yourself a copy of D. G. Hessayon's The Lawn Expert, it's not expensive, and may help you to decide quite what's going on. I don't believe differences in light levels will have caused this problem, but any remaining large root material beneath those areas might be an explanation.