It's Dog Lichen -harmless, but unpleasant to look at, especially spread over such a large area. It is a big signal the lawn needs some work because its suffering from extreme neglect. You need to get someone in to aerate it, then topdress it - its a sign of oxygen and nutrient poor soil beneath, coupled with wet conditions a lot of the time. Regular maintenance (aeration, scarification, regular feeding, topdressing) on the lawn should improve the problem. The scales of this lichen are more noticeable when the ground is wet or its raining - when it dries out, it goes papery, but swells again when moist.

Many thanks for your help, much appreciated!!