Tried fake herons / fake foxes but no luck.
Tried nets over pond but heron will still stab fish to death even tho it can't get them out of the pond.
Currently trying a low wire 'fence' (about 30cm high) all way round edge of pond and it seems to be working.
Incidentally, I have noticed that crows don't seem to like herons and will mob them, so I'm trying to encourage crows into the garden with the odd bit of meat or chick carcass on top of the garage.
Who knows?