Damaged lawn - who is liable landlord or tenant?

[Deleted User]

Did you use google at all before posting on here?  You need to kill the leatherjackets as problem will just continue.
https://www.amazon.co.uk/Nemasys-leatherjacket-killer-100sqm-pack/dp/B000VPCUPW
If it works as quickly as reviews and listing says, you probably have time to get rid of them and replant seed, particularly if you get quick growing seeds.  

Otherwise, I am afraid, if you had been keeping an eye on the lawn, you could have spotted this earlier and used other methods of getting rid of the leatherjackets (turned over soil and let birds eat their favourite food).  So it is neglect really.  Which means you are responsible, just as if you hadn't mowed it during your tenancy.

You could try discussing with the LL and see what he advises but he will probably say you will have to pay for removal of the pest and resodding the area, which won't be as cheap as the above.  Up to you though. It might be a risk worth taking.

Davesnave

JMKelley said:

Take photos of everything.
Tell the Agent/Landlord immediately of any damage and keep email evidence.
Inform the landlord of any fix you are attempting to do before doing it. (They may want changes anyway)
Got told this from a nice letting agent on how to keep most of your deposit.

Pity he wasn't a nicer one; then you'd have been able to keep all of it!

It's too late to go for biological control, but there are mechanical methods of reducing crane fly larvae and coupled with some watering and re-seeding you might have something acceptable by July.

https://www.rhs.org.uk/advice/profile?pid=651

In your defence you might say that we've had two very dry summers in succession and it looks like this will be a third.*

*Maybe not applicable in some areas, like Scottish Highlands, but then nothing is up there.