Thanks to some advice given to me on this forum I've changed Arch's supplements for his arthritis and HD. He's like a new dog!
We were out in the park today and threw his ball and he jumped in the air to catch it....a full on GSD jump in the air vertically fully extended (if you know what I mean).
I could have cried with joy. He's off his metacam and we've been alternating ball throwing sessions with ploddy walks around the park and it seems to be doing the trick.
It may not seem like much to those of you with younger dogs but for me it was a miracle
I know it won't last forever but it was an absolute joy to see him and it is such a wonderful site to see a GSD jump full off the ground.
Thanks to everyone for your suggestions

and a big thank you from Arch