Good lord yet again theres some total twaddle on this thread.
There is no DNA testing for pitbulls.
Reporting it MAY result in the dog being siezed if suspected as being a pitbull or pitbull 'type' dog.
As far as UK law is concerned, if it looks like what DEFRA define as a pitbull, according to measurements, appearance, proportions etc.
Then it is a pitbull type, and it is illegal.
Doesn't matter if its a pedigree lab x pedigree staffie, or an alien from outer space, if it LOOKS like a pitbull, it IS a pitbull.
IF the dog is seized and IF the dog is then determined by measurements to be a pitbull THEN the question is, is the dog dangerous.
If its not, it may go back to the owner IF they are a suitable person (no criminal record, suitable housing etc etc).
All of this may take months or years, during which time the dog sits in a kennels, deprived of most of the basic things a dog needs (exercise, interaction with people, play, training, toys, etc)... which can and will have an effect on behaviour and temperament, to the point where some dogs having been deemed not dangerous by the courts, have then been euthanised as their behaviour has deteriorated so much due to the neglect in kennels.
If you want them to behave better and be more careful with their dog, speak to your local dog warden.