Jo,
It's good that you have a harness for her, as that will help prevent her damaging her neck/spine when she pulls.
I don't blame you for having an extendable lead, but it's not a good idea for a puppy to be on one, as you don't have control over them and they need to learn to walk properly on a flat lead, which they won't do on an extendable one.
So I would suggest that you stick to the normal flat lead. But when you walk with her and she pulls, just stop and don't go anywhere (you need to allow plenty of time for walks when you're doing this!). Any time she pulls and you let her pull is giving her the message that pulling gets her where she wants to go, so it's important not to let her. Another thing that can he helpful, if she pulls, is stopping and turning back the other way. And then if she pulls again, stop and go back the other way again.
Have you seen the "Teaching a reliable recall" thread in the Behaviour and Training Folder? It's really really worth looking at (I used it for training Alfie).
Have you enrolled the pair of you in a puppy class at all? Because that would help too.