I have certainly come across glucosamine being used for dogs with joint problems with some success. Interested to hear about the other remedies. Although out current two are still quite young, it is always good to have extra tools in the tool box.
Gazrob - when we got Buster the vet was concerned about his patellae, although they did not cause any issues at that time. I gradually built up his exercise and so developed the muscles in his back legs. Touching all available wood- we have had no issues as the increased muscle holds the patellae better. Salmon oil aside, you may have done something similar? Either way keep up the exercise with Marley and hopefully you won't have problems.