sbiddle:
If your property is a lease hold then it's not just your property, it is also owned jointly by the neighbour. They have rights as well when it comes to both properties.
Who owns the plants on a cross lease?
Access to land is one thing. Damaging private property (grass) is quite another.
I'm fussy about my lawn and I'd have minimal confidence that it would be restored to my standards. Also confident the fibre installer wouldn't be back to water and weed the repaired section. The expense/work would fall to the owner.