There are some training techniques you can try Blodwyn. If you Google 'dogs chasing cats', there are some videos of various techniques. If the dog responds to 'leave' and you can break the urge, get him to come to you, reward, and then keep practicing, eventually it will sink in. Chasing is really pleasurable and exiting for them so you have to try and find a replacement activity that is more exiting. After 5 years ours will respond to 'leave' for a treat providing he is close to us. You can sometimes break chasing by teaching the dog to 'wait', throwing a toy but tell him to wait..and then tell him to go a fetch it. This teaches them a bit of impulse control. Give lots of rewards when they do it. (Works great in theory). https://www.petfinder.com/dogs/dog-problems/dog-chases-cat/