I am in the same position as you, we are just digging out too!
I have been scouting round the net, and most options have people who say it works, and others that say it doesn't
The plastic heron ( I so wanted one... ) seems iffy.
A net either over the pond (if not huge - ours will be reasonably modest), or around the edge (as someone I now know has) seems to be a good option. Will a net affect the plants you put in though ? I went to Maidenhead Aquatics on Monday and they also recommended this - other option was tripwires round the edge (sure he said use angling wire/thread).
Can 'Pondies' please advise what they use, I am like Steve, waiting with bated breath !
My parents have a pond and suffered from herons pinching the fish for many years. They worked out a three pronged attack which seemed to work:
1) The rather dodgy looking, but perhaps useful plastic heron by the pond. It puts herons off, as they don't like competition apparently
2) A fine net mesh over the entire pond. They had a wire fence around the pond anyway so it raised the netting above the pond, so there was room for the aquatic plants to grow and frogs and other wildlife could still access the pond through the wire fence. I know this doesn't look lovely, but herons are crafty and a partial netting doesn't necessarily deter them in my parents' experience.
3) They planted heavily around the pond - pampus grass mainly which obstructs the view for a heron flying over. Rocks and plants around the pond also provide good coverage for frogs and toads if you are wildlife inclined.
Hope this helps. It is really good to think about this now whilst setting up the pond, as there is nothing worse than losing your fish this way. As a small child I remember being devastated that all my goldies had been eaten. And then the next door neighbours gave us some of their fish, and they got eaten too. Following that massacre, my parents worked on a heron proof pond.
The last goldies died of old age very recently. My parents have decided not to re-stock and have turned the pond over to wildlife. They have a very large proportion of newts apparently
My mother and stepdad have a massive pond full of fish, they have a Heron in there almost every day sometimes two. You can tell when the Herons have been as they leave an oil slick on the water. The fish have plenty of places to hide and most survive, unfortunatly a few years ago a Heron managed to bring the KHV virus to the pond which killed loads of the fish.
This thread has already helped as one idea was to put netting on the pond, but I had not thought about frogs being able to get in and out of the pond. So thanks for pointing that out :)
One thing I was thinking about was wooden posts with thick rope draped between them, bit like the ropes you wait by at the cinema. but getting the perfect height would be pretty important.
I will try and get some photo's of the pond put here soon :)