Most of the criticism about the RSPCA on Mumsnet seems to relate to them not doing enough rather than being over-zealous
Nope thats wrong, I've posted (old user name) on dozens of threads where they've absolutely been over zealous.
In my personal experience they took a friend to court because her native ponies were in a field in winter (february) without rugs. They looked like wooly mammoths they were so hairy. They dont need rugs in the south of England when you come from the Shetland Isles and Highlands of Scotland. 
They seized her ponies and tried to prosecute on welfare grounds. The judge laughed the RSPCA out of court but they didn't give up. They tried to get money out of her for the cost of keeping her ponies during their investigation. They went back to court asking for £10,000 for up keep costs and the judge warned the RSPCA that they were close to overstepping the mark with regard to the number of non-cases they were bringing before him.
My friend didn't have to pay and to this day the RSPCA still harass her dropping in unannounced to follow up with the case which they lost in a Court of Law. Shes made dozens of complaints about the staff not leaving her alone and their defence is theyre not interested in her concerns theyre only interested in the welfare of her animals.
I could tell you dozens of similar stories some from other horsey people, some farmers, some police and some vets. They all agree the RSPCA is wasting the money donated to it and that they overstepped the mark far too many times by prosecuting the wrong people.
They seem to go for easy targets, people who need extra help providing the right care and not taking to court. Most of those they prosecute dont have the money to fight it so give in then find they have a criminal record when the reality is the RSPCA failed them by not offering any real help or support. Real genuine cruelty cases are difficult to gather evidence for so they dont pursue those as much as they should.