What amountof food should I feed my dog?

Polly a 21 week old female staff. I am currently giving her 1 tin of pedigree chum puppy food with 4 handfuls of dry mixer. She has about a handful of pedigree gravy bones throughout the day as well. She's up to date with worming.
I have noticed that shes looking quite skinny, (cant see her ribs) but she looks like she has no muscle mass, her build looks more like a chubby whippet at the moment. Is this because I am not feeding her enough?
Also does she need a coat in this weather?

First and foremost, I'd get her off anything 'Pedigree', its one of the worst foods out there on the market.

Personally, I'd move her onto a decent dry food, like Wainwrights which is fairly affordable, you can get it from Pets At Home. It'll have daily feeding guidelines on the packaging so you'll know exactly what to feed.

Regarding the coat, I'm currently looking for one for Zeus at the moment for when I take him out on walks. He's got a jumper which we put on him at night when he goes to sleep in the kitchen (which can get cold).

Pedigree is not the best quality food out there, try having a look at the recommended foods from the list below, you may want to try a sample size bag to begin with to see if your dog it. Rocky has Barking Heads TLC which I would recommend.

If you decide to change food do so gradually to avoid an upset tummy:

http://staffy-bull-terrier.co.uk/dryfood.html