I know this may appear a little extreme, but have you considered an electric fence?
I had a problem with herons and cats when I first built my pond.
I bought an ESM 400 Electric Shepherd after chatting with a local farmer.
He reckoned they were good for 20' straight up in the air for cats
Spec is:
Recommended for line strip grazing for cattle.
Open line voltage 7500v
I did a diy job. The unit cost £80 I used 10mm plastic waste pipe cut to 2' lengths, 45' bends and fitted a further 6" length facing towards the pond. I spaced the pipes 1m apart, with 3 wire lines spaced about 6" apart.
Well the cats only managed about 10' but still quite impressive!
Heron got hit once and did not return. I touched it once by mistake,
felt the jolt right up to my armpit, made my toes curl up as well
I only switch it on occasionally when a new animal comes to visit and have had no further trouble with herons or cats.
It also works on small children and inquisitive adults that don't believe I'm mad enough to put an electric fence around my pond.