Here I go again (I've just prepared this reply once and then somehow deleted it??!!!). As a NON cat-expert, I have just read a book by Vicky Halls (a cat 'counsellor'/behaviour therapist) called 'Cat Confidential' and I think it should be a must-read for all fur-babe-mums. Certainly, I agree that your first priority is a health check as aggressive behaviour can often be caused by pain/illness. After you get the all-clear, I would echo Gill's suggestion about getting Da Bird from the Purrs shop - in my opinion, the best cat toy ever invented and Percy wants me to say that he
!! This way, you can play with Dayzie at a safe distance, providing her with a more appropriate target! Throughout her book, Vicky Halls says that cats learn their behaviour as kittens, so what you are seeing now could be a result of that kittenish rough-play ...... only increased now that Dayzie is fully grown. The suggestions in the book are to ignore any 'inappropriate' behaviour - no eye contact, no sound, movement or show of emotion (difficult if you are on the receiving end of those claws?!) - as punishment is ineffective. Reward Dayzie when she is quiet and restful - but only with soothing sounds and SHORT periods of stroking (don't allow them to be long-enough so that she thinks it is time for more aggressive play). To help her fulfil her hunting instincts, start by hiding bits of dry food/treats around the place, making it more and more difficult to find as she gets used to the idea (e.g. putting it in paper bags/cardboard boxes).
I hope these suggestions help - do let us know how you and Dayzie get on. I wish you both well.