Thank you both
Rossi's back home now - fortunately the antibiotics brought his temperature down to an almost normal 39.5 by 5pm so he was allowed home without needing to go on a drip, and hasn't been sick during the day.
He's been given an anti-emetic injection as well and we've got a week's course of antibiotics plus some stuff to give him an hour before meals, so providing he isn't sick again overnight and his temperature is OK tomorrow morning, that's it as far as vet time goes.
Unfortunately there was a barking dog in the dog ward for most of the day so he wasn't happy at all about being there - on the plus side it meant he was extra-happy to be back home
What he isn't so happy about is that he is only allowed little portions of food at the moment so as to avoid a repeat performance of yesterday, so far he's kept down what he's been given and is telling me often that he's ready for some more!
As far as what caused it, the vet thinks he's probably just picked something up from the garden, maybe another cat's poo or something
Not sure what we can do to avoid that happening again other than scouring the garden for poop every day (it's a big garden with a potato patch at the end so not as easy as you might think to spot poop).
Anyway, thanks for the positive vibes - keep them up overnight please