I can assure you Cotswold lass,that it does not!! It was thought to be a deterrent but extensive studies have now shown that it may work for a time in winter as herons normally fish alone then,and because food is scarce will see another lone heron as a possible threat which may attack,but in summer they are looking for a mate and so are quite keen to investigate!! One way is to keep moving it around as herons are wily birds and will soon suss out it is a fake.
I have always had a fake heron,more of an ornament really! which has certainly not stopped visits from the real thing! The only sure way is a net and though a little unsightly,you do get used to it and it is sure better than losing all your fish.