suspect the neighbours are neglecting their cat ..

Not sure what to do here... bit of feline lovers support is needed because most of my neighbours round here ( North Yorkshire UK) hate cats - they complain about them pooing in their garden ect... our 15 year old Maine Coon was shot with a pellet gun this summer! ( he is OK and survived) But basically a neighbours cat - a female silver and white tabby called Sam who is quite elderly ( about 15) is looking increasingly neglected.. she has always been ravanously hungry and eaten anything we put out for the birds, bread ect.. and raides our bins every night. Always looked a bit moth eaten... but recently she has been turning up at our door and I have noticed she has a horrible eye infection in both eyes - and horrific bald patches on her sides with raw skin exposed. She has very dirty fur - black flea droppings I think and her teeth are terrible - very brown and she has a black lump the size of a pea on her gum? what could this be? She has black dirty ears as well. My husband went to have a word with her owners and expressed our concern and offered to take her to the vet if they were struggling to find the time, however they declined and said they would get her to the vet themselves.. a week later and she doesnt seem improved - this morning her eyes were as gunked up with nasty stuff as last week. Our own cat Kato got an eye infection and had to have treatment for it - could he have caught it from her?

I was wondering what others would do in this situation? I could call the rspca but they havent done me much good recently when I complained about some guineapigs that were severely neglected in the area - I could take Sam to the vet myself, but I cannot afford the vet fees - I have my own animals to care for. I could try approaching the neighbours again however just think we will fall out and that it wouldnt do much good for Sam anyway. Could I be over reacting and all these things just be a sign of old age? I just dont know what to do :-(