I can tell you where it gets to the fish. you provided a perfect path for it to wade into the pond. It looks great by the way, stunning would be another word for it
You need to have a sharp drop from the surrounding garden to the water, preferably vertical, and it needs to be at least 8 inches. The damn things are waders, they wade over rocky ground in the rivers and and streams to catch their food- thats probably why your pond attracts them (That and the fact they are greedy and eat whatever they can get their hands on!) If you get a sharp enough drop they are forced to sit alongside the pond and wait for a fish to swim within reach before they can strike, wich improves the chances of you seeing the damn thing and chasing it off.
If all else fails buy a dog! Cat's won't scare a heron, they are just too big. Six and a Half stone of English Bull terrier does the trick just fine :P