Destroyed Manhole Cover - who is liable?

[LGR001]

Not sure if this is the right section for this thread but here goes: A young chap came to cut my fields. He came last year and cut the fields. The grass on both occasions was quite long i.e. 12" approx. In the middle of one field is a manhole cover. I would say that someone high up in a tractor would see the manhole. Last year he saw this and avoided mowing over it. This year he completely destroyed the manhole cover and surround. The drop down to the sewer below is 22 meters!!! I therefore called out the water board who came, inspected, and repaired it. They said it was a 3rd party responsibility so I gave them the young chap's name, address and number. This young boy has just left college and started up his business last year so he must have insurance for just such an event. However, he borrowed the cutter from another chap who phoned up the NFU and they said it is a grey area as (1) I should have told him that it was there (2) The fact that he cut the fields last year and avoided it did not count at all as things could have changed in that year. (3) He should have asked before starting if there was anything he should be aware of. Would appreciate your thoughts please.