Grass eating caterpillar

Please can someone help, my lawn is being eaten by a caterpillar! There are literally hundreds of them and my lawn is not very big so its a bit urgent!!

If the caterpillar is not going to destroy everything then I will leave it to take it's course, but if I'm going to end up with nothing then I would prefer to get rid of it. But I can't decide this until I know what it is, which is where you lovely people come along.

I've attched a photo

 

Looks like cutworms to me. Which precise type doesn't matter if you have a large infestation because they are very destructive. You need to kill them.

 

Update - I have found out it is the grass eating Sawfly larvae, just in case this helps anyone else out.

 

Let them eat it! It saves mowing it lol :P