"pied piper" wrote
There is a spray called crossfire which prvents the eggs from hatching
and is safe to other wildlife in your garden.
I suggest you visit your local groundsman or greenkeeper and ask if
you can buy some from them a lawn that size u can apply it with a
watering can

The hazards info does state it's very toxic to aquatic life though, so
you'd have to be very careful near any ponds etc. I'm not totally
against using a chemical product if it's (relatively) safe and there's
no other good way, but for such a small area of grass it seems like
going way overboard just for a few leatherjackets. Is Crossfire
available to the general public? Just a thought, but if not, one might
be asking a greenkeeper to break the law by supplying it.

--
Sue