In my experience of staffs (30 years) your puppy looks like a cross to me, Staffs do not have the white ring that goes right the way around the nose,
It is a tell tale sign that you have purchased a staff/cross, I would say that as long as you paid no more than £200 (MAX) then its ok.
If you have paid more than this then you have a fight on your hands to get the balance back off the seller.
If he /she told you that it was a full staff when you bought it then you have a case with the trading standards people.
Talk to the seller be diplomatic but assertive and demand any money back over and above £200. Your better off talking to the seller amicably and dealing with it between the two of you as court cases drag on and can be time consuming. All your rights are protected If your seller is dragging his or her feet in agreeing a partial refund.
Personally, I think that staff cross dogs are worth about £150 because you cant stud the dog and if its a bitch, the puppies are worth much less than Kc registered and full breed ones.
Im sorry to be the bearer of bad news but I hope these comments help.
see Kennel club website for "The breed standard for staffordshire bull terriers"
All the best and good luck , sam0268