Thanks for your replies guys. I shall contact the vets first thing in the morning. They are normally excellent (they are an animal hospital) but Hummy has not seen this vet before, I am wondering if she may be new. She had said she was going to admit her for the X-rays etc, but then changed her mind as Humbug was freaking out with all the barking doggies (she is terrified of dogs) and she said her adrenaline was kicking in. My partner gently suggested should she perhaps be admitted for tests etc (she said she would have to be anaesthetised for bloods and X-rays) but the vet said to see how she goes 'til Monday. I am very, very glad to say she has just had a little bit of her favourite fresh tuna. She did say to contact them if we were worried, so will do this.
Thanks again - will keep you posted (am also posting this on CatChat).