OP, I hope you won't mind if I offer an opinion on Squashyhat's question?
I used to work for an equine charity, so had a bit of experience with the RS. I would say that the majority of people who work for them are as dedicated as anyone else on welfare. However, there is sometimes a lack of specific equine expertise (after all, the inspectors have to deal with everything) and like all the rest of us they are rushed off their feet. As a result things can fall through the gaps. That said they're often the only ones who will take a prosecution forwards and it's a bloody good job that they're there, IMO.
However, for horses you are better off calling an equine specific organisation. World Horse Welfare have field officers, the BHS can take welfare calls, Redwings will advise. Others do their bit too.
On the equine side there was certainly a lot more coordination when I moved away from the sector than there was when I started and I would imagine that's still the case - RSPCA very much included. And LAs too - we do try and work together where we can (IME).
Hope that helps and that you don't mind me offering a perspective, op!
Actually I have a question too - do you get much fly grazing in your area, and if so are things better or worse since the Control of Horses Act? Or no different?