There's a limit to wind speed for spraying - about 6 or 7 mph I think. Contractors should all have massive insurance, at least up to a million £s. If you don't have official organic status it would be difficult to prove any damage though.
When a similar thing happened when our neighbour's field was sprayed, again using a contractor, I stopped him in mid spray and now he keeps well away from our hedges. I lost a crop of sweetcorn one year to weedkiller spray drift and another year some fruit trees.
Our neighbour is very conscientious and was upset at what happened, although the contractor himself just airily said that he had a million £s of insurance so we could sue if we wanted. The neighbour is now careful to let us know when he will be spraying, so I can keep my polytunnel closed, and he is also careful to keep below the wind speed limits.
I know this doesn't help in your situation, except if it was me I would be contacting the contractor and getting details of his insurer. You might not be able to win but just to start a case should keep him away from your pasture and hedge in future.
I would certainly be ranting too