It is not that people in Barcelona take notice of the law or signs either, unfortunately.
However, I have not had any serious incidents with dogs up until now.
If I see a bothering dog I really shout at it to shoo it away.
On the other hand I shout at the owner: "watch out, this kite costs 250€ and if something occurs you'll have to pay for it". After a short face of incredulity and after a second warning they tend to call their dog and put it under control.
The other point is that I do tell the owners, you should really have your dog on a lead and then they answer "And kiting is not allowed her... and moreover what if you damage something with your kite?". To which I answer:
1. there is no law forbidding kite flying in public parks
2. I have a national air sports federation member card which includes an insurance in case any mishap occurs. Do you have an insurance?
They then leave with their tail between their legs and the dog on a lead...
The two worse incidents I had was a dog peeing on one of my kite bags which really got me p*ssed (!)
And the second a stupid dog that almost pees on me (!) while I was kneeling down folding my kite away. The stupid dog came from behind and lifted its foot as if I were a f*ck*ng tree!!!
I just couldn't believe it! But fortunately I scared the thing away before it left any drops out...
But then again, it is not the poor animals' fault. The owners are the ones to blame and keep under control.
So there you have a few more arguments in case you guys are also federated in the UK. Sometimes it can be powerful to waive the official card in front of the stupid offender's face...