Thought I would update you on the outcome of the visit to the specialist today. There is nothing wrong with her legs at all it is rapid growth, we have to stop feeding her puppy food immediately and put her on a good adult food, no supplements etc. She has to be exercised more and try in long grass or water which will encourage her to bend her legs. What has happened is the bones and the back muscle of her legs has grown rapidly but the front muscle and tendons haven't so she stands with them straight not bending at the knee which makes her go on tip toes. Also the tendons in her toes have not caught up yet so we need to massage these. The vet seems to think that we should start seeing improvements within the next week which is good news. At least we now know there is nothing major wrong with her legs. Thanks to everyone who gave advice and support.