Thanks for all your ideas. I've gone with the netting and little frogs have proved that they can cope having seen one sitting on top of the net! I've replaced the goldfish but haven't seen them since putting them in yesterday. I hope that they are shy and adjusting to their new environment and that the heron hasn't been back and laughed at my net.
Herons are very intelligent, my neighbour found that when she had a model heron (which looked quite realistic), she had to move it around or the real heron soon realised it was not real! Eventually it got the idea that this heron was there just too much of the time, and got all the fish anyway! They now have a rather nice bog garden and no fish - but we all have plenty of fros from my other neighbours garden pond - in which they do not try to keep fish.