Thanks for the comments guys. Netting would be something I'd like to avoid, as it would make the pond look more like a fortress than a natural object, and ruin it visually.
I've read from numerous sources that stringing some fishing wire around the edge is an effective deterrent. The herons are confused by the fact that they can't move close and go elsewhere. The irony is that I was planning on doing this yesterday, just never got around to it due to heavy rain.
My pond has rocks around the outside too (
picture here), but that didn't seem to bother the Heron. I guess they aren't high enough, and there are gaps in places. Since the pond is surounded by the house wall in three of the four directions, herons don't have much of a landing area. The one I saw a few days ago actually landed at the very end of the garden and slowly walked towards the pond (taking a good 1/2 hour to get there, before I scared it off).