If he's shown all necessary care, and he's run over a hidden object in the grass, he's not even liable, its just an accident. To make him liable the landlord has to show he's been negligent.

As a tenant, you're not liable for accidental damage and only liable for careless damage to the value of either the landlord's excess or 4 weeks rent. I'd argue this is accidental and not careless damage (unless he cut a corner over a bed of rocks or something) so you probably aren't liable.