KatieMac you can't just put it on grass because the grass will GROW through it and also be impossible to cut around the edges...
And it will be v. uneven.
You will probably need to dig down a few inches to get all the turf off, then lay down some membrane so new grass doesn't grow through, then dump, say, 3 tonnes of gravel on it. (This is what I did to mine.) This is the easiest way of doing it I reckon.
Wickes are good for advice though - I'd recommend them.