Laptop wont charge properly & never lasts long, new battery and charger replaced?

heya, im after a bit of help with my laptop, im a girl and no knowing about laptops but mine wont charge properlly, was purchased 3years ago from pc world for �499 and was used a lot, the charger stopped working but i replaced both the battery and the charger but still got the same problems, i have to really wriggle the charger around for the laptop to work so im guessing a wire is lose, but also the laptop doesnt stay on for more than 20mins, i was going to pay for it to be looked at and fixed at pc world, but not sure if that will help? any help apreciated, laura x