Dog fouling and dumping of pooh bags in the alley

There's a dog fouling sign opposite my property 173 Sandy Lane indicating a fine will be issues to offenders. Nobody has ever been fined and I have CCTV footage of regular dog walkers picking up dirt in a bag and then flinging the bag in the alleyway next to 171 Sandy Lane. It happens several times a day. What do I do about it? How can I get people fined? Who's enforcing the fines? Have people got to be caught by a police officer to BE fined? Who's responsible for picking up after these scumbags? It's been going on for years because the alley gate does not go right to the top of the last property of St Peters Road and there's an area that's regularly being dumped by passers by. It's been a problem for 30 years I'm absolutely sick of it