You can't be sued under the Health and Safety at Work Act, there is no legal mechanism to do it. There may be a civil case to do with negligence if someone fell off one of these bridges and could show that it was genuinely dangerous, and the farmer (or whoever) had a duty to have it there for whatever reason.
Similarly, if the bridge was there AS PART OF THE BUSINESS UNDERTAKING of the farmer, it would have to conform with all sorts of legislation, but basically be fit for purpose, i.e. take forseeable weights, not be too slippy, clearly visible edges in the gloom and not deteriorate faster than the inspection regime. If it wasn't part of the business undertaking, then not a single word of the H&S legislation applies... the clue is in the title 'Health and Safety at Work Act'.