Lawn Weed Identification please

Hi, please could someone give me some advice on how to deal with these, which have appeared all over my lawn.
Thank you

 

They are tree seedlings, probably from a tree nearby. Simply mow the lawn, they are unlikely to survive decapitation.

 

thank you both very much

 

MURDERERS!! Lol.

This takes me back to my childhood. I was always fascinated with the sycamore tree "helicopters". I used to take them home and I would grow them in pots. We lived on a smallholding which had about 6 acres with it.

Once the seedlings outgrew the pots, which was soon, I planted the young saplings at the side of walls and other sheltered places around the farm. By the time I flew the nest years later a lot of the young saplings had grown into sizeable trees.

Fast forward to the present day and once in a while i'll have a walk up to the old farmhouse. A lot has changed and some of the trees have gone making way for buildings and such. But the odd one still remains. The biggest is around 30ft tall!