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Kita 1 - thanks for that link. I've had a look at it and it is very good at explaining the reasons for chasing and why it is so hard to stop. (I particularly liked this analogy:
'What is your favourite exhilarating activity? Hang gliding, ballroom dancing, cuddling your grandchildren, alligator wrestling, strip scrabble, or extreme ironing? Imagine you are halfway through and I say, “Stop that now and I’ll give you a biscuit.”
Would you?
No, and neither will your dog.'
which makes perfect sense!). I took her for a long walk earlier, callling her back frequently, and each time she responded perfectly - there were no deer or hares around, of course. So the point that the dog doesn't hear when it is chasing is reinforced by this - she does hear when she's not otherwise distracted.
I like the whistle idea, but will have to try it out with the horses first because they might not be too impressed. Meanwhile I shall have to try tempting her with a ball or something, but as she is not really interested in balls I shall have to be inventive. Fortunately she doesn't chase cars (she's more afraid of them) and confines her chasing to game.
Thanks to all for your suggestions.