Just in case it jogs anyone's memories, here are the few further details I have.
"Fish" was about the developing friendship between two ten year old boys in rural Wales (at least it was Wales in the book) - one a farmer's son, one a lonely and awkward newcomer to the area, nicknamed "Fish". It was a realistic story of very ordinary early 70s boys who played football, wore anoraks and spent their spare time idling mucking around - just like me, which I suppose is one of the reasons I liked it so much.
The narrative revolves around the appearance of a stray dog, which is taken in and adored by Fish. When the dog is suspected of sheep-killing, Fish runs away on the day he is meant to take the dog to the vets to be put down. The farm boy helps him hide away in barns and woods for a week while the police are searching for him, ncreasingly convinced he's dead and eventually arresting his own father on suspicion. Both boys are finally stranded in a remote dereclict cottage for two days before they are found and all is resolved.
I recall it as being very realistic with a truthful sense of the gulf between the adult world and that of the boys. There was a real sense of peril and the presence of the darker undercurrents of adult life, which I think had a big impression on me, even though I was too young to fully grasp it. But I was old enough to know it existed and Fish was the first children's drama I saw which acknowledged this.