Look at the Cats Protection website.They do not prosecute like the RSPCA but they provide info on the 5 welfare needs that must be met by cat owners who otherwise can be legally charged with cruelty or neglect.
Like everyone else I am appalled by your neighbour's lack of care in this respect.
I appreciate what has been said about the RSPCA though - our own experience of this organisation is not nearly as positive as that of CP.
You could perhaps let the neighbour know that they COULD be reported by someone (other than you!) Or risk losing the animal if it decides to find another, more reliable home.And then offer to feed it if they pre pay your costs. Maybe even take it to the vet if it suffers an injury, as long as they have agreed with the vet beforehand that they will pick up the cost of this too....
We have always had cats.Including one that adopted us after suffering cruelty from a neighbour' s child.And another that was ill treated and dumped in our village, and had no idea how to go and find food for himself.He camped in our garden for over 6 months before he was sure we were not going to hurt him, and then joined the rest of the group indoors as well.
We have happily used catteries where our cats have been well looked after.Lately, we have had a friend call in twice daily to feed/ water and look after them, and change their litter trays ( they are kept indoors for their safety, but are given plenty of room to run about).This has the added advantage that the house is kept an eye on, too.We pay this friend,the going rate a cattery would charge, plus her petrol costs.It is worth every penny as far as we are concerned.