Thankyou all for your kind thoughts .

Good news - we think the mystery is solved!

We've been keeping detailed notes on her food and water intake, as well as when she is sick. When we showed them to the vet this morning he noticed a pattern. She seems to be getting sick after eating one of her favourite (make that ex-favourite!) foods - seafood and chicken mornay. He thinks she has developed an allergy to it. This explains why she wasn't sick in hospital. It's now been 12 hours since she last vomited and she's managed to eat about 100g of food in that time (a nice beef dish!).