dogs roaming into private land

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there are some walkers whos dogs are not on a lead, the dogs keep coming onto my land and worrying my lifestock giving chase to ducks geese hens and horses, the dogs i have here are working dogs i.e guard dogs, which chased the dogs that had tresspassed and a dog fight broke loose which i broke up and brought the situation to a stop i asked the walkers 5 times to get their dogs under control which they choose to ignore. The next day the police came as these walkers reported my dogs and said they were let out purposly to harm theirs , my dogs are kept in an enclosed yard secured with five foot gates which they never escape and their dogs entered my premisises. Where do i stand legally with there dogs tresspasing. many thanks viv stainer