Sean wrote:
The lawn has been laid over a huge amount of debris left by the
builders of this new house I also believe this may contain Laurel
Bushes and similar. your answer makes very good sense.

Ah yes, the classic new home construction type soil. I unfortunately
know that kind of soil all too well. Expect mushrooms for quite a few
years to come.

They are a somewhat good sign, as they indicate good natural processes
are going on underground to help the old wood decompose.

I went out with my string trimmer yesterday. We had had about a week
with rain almost every day, and the grass was very tall, still fairly
wet, and had lots of mushrooms of all shapes & forms. The weedwacker
made short work of them, and splattered me with mushroom muck on
numerous occasions.

David in MD zone 7