The only time I've seen something like this, it was all in one area outside the French doors of a garden, and it was because the husband had been nipping outside to pee there during the winter rather than going upstairs. Never seen it before otherwise. Have you fed the grass in the last 8 months at all, and if so, what with? Was the lawn laid from turf or seeded?
A closer shot of one or two of the grassy mounds would be useful to check whether they're simply clumps of weed grass.

Just a thought, and probably totally irrelevant, but these patches can often be linked to soil disturbance long before your lawn was there, for example the post holes of old wooden buildings or the sites of trees which have been removed a long time ago. This disturbance is not visible at the surface but the soil profile is permanently changed and so anything growing in these spots may grow differently.

I have one patch in my bottom lawn that grows like this, Interested to see any answers.

Thanks for the initial thoughts.

The lawn was laid from turf, bought from a reputable source. It has not been fed because the gardner recommended that we should not do that until this year (it was laid in March last year). It has been scarified recently, about 3 weeks ago, and the patches were not evident then. I can discount the soil disturbance because we have totally relandscaped this garden and the new lawn was dug down about 3 feet to allow the sinking of water pipes and electricity. The old lawn did not have this issue. Also, I can discount peeing on it, honestly I can. However, we are visited by foxes, could their "deposits" being playing a part?
As it continues to pour with rain I shall defer getting a closer shot of a patch of grass for a while, but I can vouch that there are no visible weeds or fungus, the grass appears to be healthy, just longer and darker than the rest.

Thanks again everyone, any more ideas?

The only explanation I can think of is the presence of weed grass - this grows faster than lawn grass and makes clumps like this. Why they're there is a mystery, though, if the ground was prepared properly before laying, and its only appeared this year. A close up view might confirm this theory. What does your gardener say about them?

interesting, just wondered have you not cut these clumps when you cut the lawn? If you have, for some reason you have very rapid growth in these areas. The clumps appear to be completely different from the rest of the lawn. In my experience " weed grasses " grow much faster than cultivated grass types but i have to say, they don't look to be " weed grass. " The only thing i can think of is they are just a much more tolerent type of grass for the conditions in which the lawn grows in.

Not really close up enough to be sure whether its weed grass or not - the commonest one is known as Yorkshire Fog, but I can't tell whether the blades on the clump are slightly wider than the rest of the lawn. I can, though, see the leaves of some invader (broadleaf) to the left of centre of the clump in the picture. When did you last cut the grass? I wouldn't have expected any invaders, particularly not broadleaf ones, in a lawn that's only been laid a year, unless they were in the turf when you got it, or are coming up from beneath the turves where preparation wasn't perfect.