Zara was walking great on the lead when she was little but she broke her back leg and was in cast for almost two months. She was only allowed out to the garden to do her business and back in again. Now she is 8 months old, very strong and pulls on the lead. I have tried a Halti but she got used to that. Zara is good natured but if she see my son out playing football I end up hurting myself holding her back....Please help as I am only 7st and she is getting bigger and stronger every day...

Here is what I would do. I will warn you it takes a lot of patience. When You are out on a walk and he starts to pull as soon as he does stop and command him to sit. If he sits then reward him with a small treat that he rarely gets but totally adores. I do clicker training so what I would do is as soon as he would sit I would click the clicker and then give the reward. If he doesn't sit then put him in the sit and then reward. Just don't move from the spot. Even if it takes 15 to 20 minutes. Keep trying to get him to sit. If he uses up his alloted play and walk time in this manner then turn him around and start going home. Basically you are saying to him that I am in control and if you want to freak out them we go no where or we go home. He will learn. Persistance is the key but you have to be very diligent in not giving in no matter how badly you want to. Now you can take a few steps forward. If he does the behaviours agian then do the whole thing over agian. He should quickly learn that the more he reacts badly the slower things go or basically stop. You can't give in to the behaviour at all.  Sometimes I would spend several days looking funny by just getting out of my yard and not looking like I was going very far.  But everyone of the rescue dogs I have fostered have gone on to new homes and the biggest compliments I get is how well they walk on a lose lead.  Try it.  It will get frustrating I can promise you that but it will work.  I hope this helps.

I totally agree with that advice.
One thing I would add though, because an 8 month gsd needs so much exercise it might not be possible to turn and walk back.
Could you have the dog wearing a halti or a harness for the walk to the park until she's learned to walk nicely on a flat collar?
I sometimes think it's kinder to teach walking nicely on a lead away from distractions first, so that when you stop on the way to the park the dog has some idea why she's stopping, and should speed up the learning a bit. Otherwise they can totally freak out and think 'why are we stopping? I need to get there!!' which makes it much harder for them to learn.
Poor dog though being restricted for so long, I don't blame her for wanting to get out there and have a good run!
Jo