My cat is a fussy little thing.

I did a food shop at Tesco's yesterday and stocked up on cat food, but she's decided that she wont eat it anymore. She had a couple of mouthfuls yesterday evening but wont touch it. I've cleaned her plate, opened a fresh tray & she still wont eat it :( She managed to pinch some of the dogs food earlier, but only to lick the gravy off the chunks (great - she likes dog gravy but wont touch cat gravy!). 

I am 100% sure this is pure & simple fussiness and would bet my life that if I offered her Sheba she couldn't eat it quick enough. She does want feeding as she's doing her usual trick of wandering around the kitchen, scratting at cupboards, smacking the unopened trays of cat food off the counter and being a general pain. 

Unfortunately, I usually give in and get her a different flavour/brand so realise I have 'trained' her to be this fussy, but I have a weeks supply here and would rather she just ate it! How long is it OK to let a cat go without eating? If it were a dog being fussy I'd not be worrying, but she's such a tiny little thing. She isn't fat, so has not got much in the way of reserves (probably ideal weight for her frame or just under ideal)

As she usually eats wet food, she doesn't drink water (only ever seen her drink water after she'd eaten dry food, but she's not overly fussed on that) - will she drink if she needs to if she's not getting her water from her diet? Do I give in and get her something I know she'll scoff? She's camped out next to her food bowl waiting for something better to be offered!

ETA the comment about her not liking cat gravy would have made more sense had I left in her entire life's history of faddyness! - the basics = she won't eat gravy/jelly cat food, nor fish flavoured cat food unless it's Sheba!