By Betty1
United Kingdom 
I have been spiking the lawn and in some places immediately some brown lavae have been wiggling out they are about 2-3 cm
The lawn look pretty healthy although I have scarified it and then fertilised it help Liz Roberts
15 Apr, 2016
Answers
Most likely leatherjackets, the larvae of crane flies, or less likely chafer grubs, which look creamy white. RHS link below re leatherjackets - I have to say the pic they've got there shows something that looks slate grey, but usually, they're more of a browny colour, but if you think you'll recognise what you saw, its worth googling leatherjacket larvae and selecting images to see the variations in colour
https://www.rhs.org.uk/advice/profile?pid=651
15 Apr, 2016
Oops, posted at the same time, Owdboggy, same link too!
15 Apr, 2016
Great thinks mind alike!
15 Apr, 2016
;-)
16 Apr, 2016
Definitely crane fly larvae I would think. Our lawn is full of them, and the garden when you dig up the luxurious buttercups that grow on our clay! Birds love them, if you have chickens they will love you for ever.
We get woodpeckers, both green and spotted, which spend hours poking in the grass for them.
They do live on plant roots I think.
17 Apr, 2016
Larva of crane flies, Leatherjackets are a possibility.
https://www.rhs.org.uk/advice/profile?pid=651
15 Apr, 2016