Yep, all I can think of is calling the various animal welfare and specific equine welfare groups - but there's little that can really be done. The police and RSPCA are both shockingly ineffective at prosecuting and/or rescuing horses from these situations over here, and there are a number of ongoing cases where the abuse has been obvious and horrific and yet they are still just going through the motions (if that). People report and report and report and nothing ever gets done. Unfortunately the police are much more diligent when it comes to theft so removing them from the property without court permission isn't an option.
Hope they eventually do something about these poor horses, but sadly I can't see anything being done in the near future
ETA: The one "organisation" that may
help is the local council (provided they're not in the pocket of said horse owners). Let them know how you feel about it and particularly stress the fact that they really devalue the area (something councils hate to hear). If you can get into the ear of someone a bit higher up, rather than at reception, then all the better. Try writing to the mayor and other councillors. There are sometimes little by-laws at council level written in about the standard of care for livestock and pets kept on private properties which, if council had a good enough incentive, could help matters.
And the other option would be to check if the property is owned by the people or just rented - if it's rented I'd contact the agent and ask them to let the owner know of the dreadful state of things (especially if the property itself is also not being kept in good nick).