Heron are amazing birds... just a bit of a pain if you've got a pond. Try the following;
A net. Also handy to stop those leaves from sinking to the bottom in winter. You can get them in different thicknesses so some are more visible than others.
A solid mesh. These are one step up from nets but rather annoying to install if you've got a big or odd shaped pond. They're also rather ugly.
A heron alarm. A high pitch screach is emitted everytime it is triggered (It has a motion sensor). Some people find that these are completly useless.
Heron Wire. If you put wire around the edge of the pond, raised slightly form the ground, the horns can't stand on the side and eat (or something like that). Doesn't really work if you have a shallow pond because they'll just sit in the pond to eat. You can get electric versions of these.
A plastic heron. Personally I think these are the ugliest option but some people love them. The reason the *shoul* (but not always) work is because a heron flying over will see another one and find a different feeding spot... Guess they aren't friendly guys.
I suggest you try one you fancy, if it doesn't work, add something else. The more detterent the better but you never know... just using the one might work against your heron chum