All over my lawn! What are these?

Hi all,

These are all over my lawn. Literally hundreds of them and seemed to have sprung up between the new year and now.





There are quite a few trees overlooking the garden if it helps, but we've never had this problem before, this is the first time.

What are they? How do I get rid of them? They look ugly and ruin the outlook of my lawn and I really want to get rid of them.
Will mowing over them help or will I have to individually pick every one of them?

Thanks in advance